P-31 MAS NMR of adsorbed trimethylphosphine (TMP) has been used to characterize the surface acid properties of gamma-alumina and chlorinated gamma-alumina. Combined with thermodesorption of TMP, NMR has allowed the determination of the strength distribution of Bronsted and Lewis acid sites on these samples. gamma-alumina chlorination with 1,2-dichloropropane at 500 degrees C first increases the numbers of "strong" Lewis and "strong" Bronsted acid sites, second decreases the number of "weak" Lewis acid sites. The "strong" sites are the ones TMP is desorbed from above 200 degrees C and the "weak" sites up to 200 degrees C.