It has been shown that it is necessary to carefully control the reaction conditions when silanising a zeolite. It has previously been shown that using vapour or liquid deposition procedures and static or flow systems influences the properties of the silanised zeolite. Here it is shown that the deposition temperature and the number of silanisation/calcination cycles are of great importance. By careful control of the number of cycles it is possible to systematically modify the diffusional properties of the zeolite while at the same time inertising the external surface acidity. On the basis of the changes in adsorption properties of the silanised zeolite, it is suggested that diffusional changes are more likely due to blockage of pore entrances, resulting in a greater diffusion pathway, than due to a controlled narrowing of the pore openings. The amount of Si deposited/nm(2) is an indicator of the efficiency of the silanisation process. A reaction pathway for the silanisation process is proposed as well as the influence of this procedure on the selectivity of the zeolite for a variety of reactions. (C) 2002 Elsevier Science B.V. All rights reserved.