A comparative study of isomerization of n-butane over bifunctional catalysts based on zeolites with different pore geometry (ZSM-5 and mordenite) and with metal particles located inside and/or outside the zeolite main channel system has been carried out. 1,1,1,3,3,3-hexamethyldisilazane was used in order to obtain catalysts with external Bronsted acid sites poisoned. TPDA, FTIR, TPR and TEM techniques were used to characterize the catalysts. ZSM-5 catalysts were quite resistant to poisoning, mainly when metal sites were located inside the pore system. However, isomerization activity was virtually suppressed for the poisoned mordenite catalysts. It is proposed that the isomerization via C-8 formation cannot take place inside the one-dimensional pore system of mesopore-free mordenite catalysts, but this is nor the case for isomerization on ZSM-5.