My personal (biased) opinion is that Snape is an  INTJ.  Biased because I'm an INTJ (and I see that you are as well.)

 I've been studying MBTI off and on for a couple years, so here are my explainations: The main reason is that I think Snape is way to independant to be ISTJ. ISTJ's are fond of doing things for a group or others, as in "being of service."  True, he works for Dumbledore, but you know that he's really just going to go by his own judgement.  If he was a true ISTJ, he'd still be working for Vodlemort, is what I think.

So here's the way you answer that question:

If he is ISTJ that makes him an SJ temperament -- does that fit him?

If he is an INTJ that makes him an NT temperament -- does that fit him more?

Ok, I just read through all the Myers-Briggs stuff on your site.  INTJ and ISTJ are really very close, aren't they?  The main difference that jumps out at me is ISTJs seem more concerned about being useful to other people, where INTJs don't seem to care so much. So I'd say that  I, too, have some of both in me.  On the test I was 82% I and N, 47% T and J, if that tells you anything. 

You mentioned that you weren't sure whether Snape is INTJ or ISTJ.  I believe that he is INTJ.  While reading the profile, it says that a ISTJ is "quiet, honest, resistant to change"  That doesn't sound like Snape.  I admit, he is careful, avoids people and so on but I truely can't see his needing time to adjust.  I doubt Tom would give it to him even if he needed it.

I can see him as an INTJ because that is exactly who I imagine Snape to be.  When reading the two "Recognising an INTJ/ISTJ" for the INTJ, I can see a hard man, who will study things but let people know his opinion and not be afraid to say what he wants to say, despite who will be offended.  He will have a "I know what I'm talking about so you'd better listen if you know what's good for you" attitude.  While the ISTJ is a rather uncertain person.  Uncertain of how his ideas will go over and uncertain if he should share them or keep them to himself.

But this is of course my opinion. 

Now, according to my research on Meyers-Briggs Type, Professor Snape is most definitely an INTJ.

Please note that the following information has been taken from the writings of David Keirsey, a reliable source in the field of MBTI personality types. (I would highly recommend reading his book _Please Understand Me II_ if you haven't already). His website is http://keirsey.com/

Reasons why Snape is an INTJ:

1. Rationals (NT personalities) are "utilitarian in implementing goals" (http://keirsey.com/personality/nt.html).  This means that they are willing to step on some toes to get where they want, a rather Slytherin trait if I may say so myself. :)

2. Also because of this utilitarianism, Rationals usually won't obey a rule 'just because it is a rule'. If they don't believe in said rule, they will most likely ignore it.

3. "Theories which cannot be made to work are quickly discarded by Masterminds [INTJs]. On the other hand, Masterminds can be quite ruthless in implementing effective ideas, seldom counting personal cost

in terms of time and energy."

(http://keirsey.com/personality/ntij.html). Professor Snape shows this when he 'discards' the students that are not up to his standards. Also, 'implements the effective idea' of him being a spy despite the personal cost involved.

4. Keirsey says of INTJ's "they tend to be much more self-confident than the rest, having, for obscure reasons, developed a very strong will."

(http://keirsey.com/personality/ntij.html). Snape certainly shows confidence when teaching the students, dealing with the "bad people", ect. and he certainly has a strong will! (Please note that this does not mean that he is always self-confident. Everyone has insecurities, just some hide them better than others.)

5. INTJ's have very good organizing capabilities, and "As the organizing capabilities the Masterminds increase so does their inclination to take charge of whatever is going on." (http://keirsey.com/personality/ntij.html). Professor Snape shows very good organizing capabilities (organizing his potion ingredients, ect.) and does indeed have an inclination to take charge of whatever is going on.

6. Said Keirsey of Rationals: "Ever in search of knowledge, this is the "Knowledge Seeking Personality" -- trusting in reason and hungering for achievement." (http://keirsey.com/personality/nt.html). Your description of Snape certainly fits this. :)

7. As a general rule, Rationals want to be independant, feeling that they are 'weak' if they must depend on others. (Keirsey puts this so much better in his book. If you are interested, you really should read it. He's a Rational himself.) They typically do not believe in conforming--especially to the rest of society--unless they believe it would be logical and useful to them.

Reasons why Snape is not an ISTJ:

1. Guardians (SJ personalities) are "cooperative in implementing goals." (http://keirsey.com/personality/sj.html). Snape cooperative?  

2. A Guardian Snape would have serveral hesitations about joining the Death Eaters. Guardians are basically sticklers for rules and regulations. In their eyes, if it's a rule, it must be followed. It certainly would be 'against the rules' to join Voldemort, now wouldn't it? :) Also, morals are extremely important to Guardians. It is highly unlikely a Guardian would go against what society generally thinks it is wrong. (Guardians also care a great deal about their appearance to society.) Snape certainly doesn't appear to care all that much what society thinks of him and his actions.

3. "Whether at home or at work, Inspectors [ISTJs] are nothing if not dependable, particularly when it comes to examining the people and products they are responsible for—quietly seeing to it that uniform quality is maintained, and that those around them uphold certain standards of attitude and conduct."

(http://keirsey.com/personality/sjit.html). Professor Snape doesn't exactly 'quietly see to it that uniform quality is maintained'. He pushes people to their limits (which will probably be very different for every person), and not in a very quiet way either. And he certainly upholds certain standards for his students, those standards are more focused on their performance than attitude and conduct--though he has high standards for those as well.

So that is why I believe Professor Snape is undeniably an INTJ. Since you are an INTJ as well, I hope I didn't say anything that might have offended you. If I did, it was with completely unintentional and without any malice intended towards either NTs or INTJs. In fact, I happen to be a fan of NTs. My favorite teacher is an INTP (as far as I can guess). As far as the other groups go, the entire rest of my family is SJ, one of my best friends is SP, and I am NF.

I hope this information helped some!

an INFJ Snape fan

P.S. I saw this comparison to MBTI and the Hogwarts houses on the Keirsey web site and thought it was very interesting. The only problem is that, according to this, (assuming he /is/ an NT) Snape would be a Ravenclaw!

http://keirsey.com/sortinghat.html