Si-29, Al-27 and Na-23 MAS NMR techniques are used to investigate Y-type zeolites before and after reaction with CFCs (CCl4, CCl3F, CCl2F2, CClF3, CF4) and a HCFC (CHClF2). Al-27 NMR spectra prove the existence of an intermediate aluminum species (distorted tetrahedral or pentacoordinated) at about 30 ppm starting from tetrahedral framework position and leading to octahedral extra framework position during the dealumination process. The presence of C-Cl bond is essential for the decomposition of CFCs, but two chlorine atoms are required for the formation of effective phosgene intermediate. HY zeolite is the least resistant to the proposed CFC compounds. On the bases of multinuclear NMR results a mechanism is given for the decomposition of CFCs. The reaction of the HCFC with zeolites follows a different mechanism. In this reaction the Y-type zeolites with basic centers are also active.