Is there an STL specification? Why doesn't putback() work?

Are there specs for STL implementation behaviors? How would I find them (preferably online for free)?

(I ask because g++'s version of std::istream & std::istream::putback(char) does funny things under certain circumstances when MSVC++6 behaves reasonably, and I'm not sure if there's a bug in g++, or if I'm simply expecting too much from it. Specifically, g++ behaves nonsensically when the character that you putback is not the same character as you last got.)