The hydroisomerization of n-dodecane was investigated over Pt supported on unmodified ZSM-22 and on modified ZSM-22. The effect of ZSM-22 modification on Pt/ZSM-22 acidity and on catalytic performance is discussed in detail using MAS NMR and FT-IR results. The ZSM-22 was modified by NH 4 + ion-exchange, HNO 3 or (NH 4 ) 2 SiF 6 treatment, and by steam treatment in order to tune the acidity of the catalysts. The unmodified ZSM-22 catalyst shows very low activity in the hydroisomerization of n-dodecane due to the lowest acidity. The conversions of the catalysts for the reaction of n-dodecane are increased from 1.1% to 96.6% at 300°C as the acidity of the ZSM-22 is increased from 0.043mmol/g to 0.329mmol/g, but the isomerization selectivity is reduced. The catalyst that was modified by the combination of NH 4 + ion-exchange and (NH 4 ) 2 SiF 6 treatment possesses the Brönsted acid sites with medium strength and shows the high isododecane selectivity (88.0%) under high conversion (87.5%) at 300°C.