How are temp tables stored?

Does anyone know how temporary tables are stored in Informix? I have heard that they are stored in memory if they are small enough, although it seems to me they would have to write out to disk beyond a certain size. Still their performance does not seem any better than regular tables, once they are created and have indexes in place. So I suspect they are really stored on disk anyway. Anybody know? I am using Informix 5, but would also like to know how 6 does it, if different. Thanks heaps Gerry Tynen