The effect of barium modification of ultrastable HY zeolite on its acidic properties was investigated using different techniques. Temperature-programmed desorption (TPD) of ammonia results showed the presence of only weak acid sites at 287 degrees C in HY zeolite. Because of barium modification, the number of acidic sites decreased but the strength of these sites was higher than purer HY zeolite. Magic angle spin nuclear magnetic resonance (MAS NMR) spectra indicated the reduction of acidity after barium impregnation. Reduction of the number of acid sites because of barium modification was also confirmed by the n-hexane cracking reaction test.