The effect of sodium poisoning on the activity for n-hexane cracking over H-ZSM-5, H-beta, and dealuminated H-Y zeolites was compared with spectroscopic changes in the region of the stretching vibration of the surface hydroxyls. For all zeolites, there was a high correlation between the n-hexane cracking activity and the intensity of the IR bands assigned to strongly acidic hydroxyl bands. The lethal sodium doses for H-B and H-ZSM-5 indicate that about 80% of the framework aluminum of these zeolites gives rise to catalytically active sites. For the dealuminated H-Y sample, only 15% of the framework aluminum results in catalytically active sites.