The surface acidic properties of two series of samples, gamma-Al2O3 and gamma-Al2O3-SnO2 after reaction with CCl2F2/H-2 (CFC12/H-2), have been investigated by solid state high resolution CP/MAS 31-PNMR, using trimethylphosphine (TMP) as a probe molecule. It was found after reaction, that Bronsted acid sites were formed on the gamma-Al2O3 surface. The longer the reaction time, the more rigidly TMP bonded to the acid sites. For the gamma-Al2O3-SnO2 system, Bronsted acid sites were also found on both the Al2O3 and SnO2 surfaces after reaction of the gamma-Al2O3-SnO2 system with CFC12/H-2. The signal intensity relevant to these sites, indicates that the SnO2 component is attached to, and therefore covers Bronsted sites of gamma-Al2O3. Two types of Lewis acid site initially present on SnO2 were not observed after reaction with CFC12/H-2.