The hydrocracking and hydroisomerization of rt-octane, 2,5-dimethylhexane and 2,2,4-trimethylpentane on Ni-containing ZSM-5, mordenite and beta catalysts was investigated at 20 bar hydrogen pressure and a temperature of 533 K. The activity decreased in the sequence ZSM-5>>BETA approximate to MOR for the n-octane conversion and increased in this sequence for the conversion of 2,5-dimethylhexane and 2,2,4-trimethylpentane. The selectivity for isomerization of n-octane and 2,,5-dimethylhexane was the highest on NiHBETA and the lowest on NiHZSM-5. The trends in the activity and selectivity were explained by the accessibility of the acid sites, estimated by adsorption of probe molecules followed by IR spectroscopy, and by a simulation of the space available in the pores of these zeolites.