I can't install a os x Mountain Lion because I have os x Mavericks?

As many others, I updated my os x to Mavericks when it was published. However, I don't like it. I downloaded a new Mountain Lion copy (through App Store), it won't install, and I cannot find it in my archives.. Any advice????? Is it not possible to go back from Mavericks to Mountain Lion???

Best

S. Vosbein