Tetrachloro-1,2-benzoquinone (o-chloranyl) has been used as a probe molecule to examine its interaction with catalytically active basic sites of cesium modified X and Y zeolites. Applying CW and pulsed EPR we were able to detect for the first time two paramagnetic species formed in the electron transfer reaction of active sites with o-chloranyl. One species is an anion-radical of o-chloranyl trapped in zeolite cage in the surrounding of alkaline ions. The other one is located on the zeolite lattice and comprises a center with a hole localized on oxygen(s) adjacent to the aluminum carrying about 50% of unpaired spin density.