The effect of alkali cations on the properties of the acidic OH groups of H-ZSM-5 was studied by different methods. The temperature dependence of the line-width of the H-1 MAS NMR signal attributed to acidic protons is greatly affected by substituting only 1% of Na+ or NH4+ ions for protons. The line-width broadens significantly upon raising the temperature of measurement from room temperature to 373 K. The line-broadening was almost completely suppressed when only 1% of protons was replaced by Na+ or NH4+ ions. The rate of H-D exchange between acidic OH groups and D-2 molecules in the gas phase is also greatly suppressed by a small introduction of Na+ ions by ion exchange, though the whole of the protons behave homogeneously for the exchange. These results indicate the presence of a long-range interaction between alkali metal cations and the remaining acidic OH groups. The catalytic activities for the acid-catalyzed reactions of cyclopropane isomerization and hexane cracking are reduced greatly when a small percentage of protons is replaced by alkali cations. K+ ions are a more effective poison than Na+ ions. These results further support the view that alkali cations in H-ZSM-5 exert a long-range inductive effect on the activity of the acidic OH groups.