The cracking of n-heptane on H-ZSM-5 zeolite was studied using a tubular reactor at 543 K, 2.4 MPa total pressure and varying hydrogen and n-heptane partial pressures. The rate of reaction was determined at times on stream ranging from 0.5 to 62 min. Under the above experimental conditions H-ZSM-5 deactivates rapidly during the first 10 min of reaction time. The initial rate of n-heptane cracking is independent of the n-heptane partial pressure between 0.25 and 0.47 MPa, while it increases linearly with increasing hydrogen partial pressure. This is interpreted with the assumption that under these reaction conditions the desorption of adsorbed intermediates is the rate determining step and that the rate of desorption can be increased by hydrogen via hydride transfer from molecular hydrogen, producing a high paraffin/olefin ratio in the reaction products. In addition to this effect, hydrogen diminishes the rate of catalyst coking.