Molecular dynamics techniques have been used to simulate the diffusion of n-heptane and 2-methylhexane in purely siliceous ITQ-1 zeolite, a microporous solid with two independent channel systems of 10 MR (interconnected sinusoidal channels) and 12 MR (interconnected large cavities). Two hundred picosecond runs at a temperature of 450 K were performed in order to characterize the diffusion features of these hydrocarbons. In the 10 MR sinusoidal system, the n-heptane diffuses rather freely through the channels whereas the 2-methylhexane is stuck at the pore entrances. Diffusion through the 12 MR large cavities is dominated by intracage motion due to the presence of preferential minimum energy positions at the top of the 12 MR pockets. Implications for reactivity and selectivity in cracking and isomerization reactions are drawn from the results.