The influence of the zeolite acidity on the kinetics of the hydro-isomerisation and hydrocracking of n-octane was studied using a refined lumped adsorption-reaction model. Catalytic experiments were performed with two Pt/USY catalysts with different composition at several temperatures and hydrogen to hydrocarbon ratios. Although pronounced differences in activity between both catalysts were observed, the selectivity for isomerisation and cracked product formation was identical. The catalytic. data were modelled using a reaction model which accounts for the different types of beta-scission. The physisorbed concentrations were related to the gas phase concentrations using independently determined adsorption parameters. Kinetic parameters and activation energies were obtained by fitting of the model equations to the experimental data. Use of the concept of standard alkene protonation enthalpy [J. Catal. 202 (2001) 324] allows to explain the activity/selectivity/acidity relationship. (C) 2002 Elsevier Science B.V. All rights reserved.