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 NH4+ ion-exchange, HNO3 or (NH4)(2)SiF6 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 degrees C as the acidity of the ZSM-22 is increased from 0.043 mmol/g to 0.329 mmol/g, but the isomerization selectivity is reduced. The catalyst that was modified by the combination of NH4+ ion-exchange and (NH4)2SiF6 treatment possesses the Bronsted acid sites with medium strength and shows the high isododecane selectivity (88.0%) under high conversion (87.5%) at 300 degrees C. (C) 2007 Elsevier B.V. All rights reserved.