Silanation, which is the chemisorption of silane, allowed modification of the acidity and the pore size of HY zeolites. Nitrous oxide was used to stabilize the remaining SiH bonds after silanation by converting them into SiOH groups. In the case of 3-methylpentane cracking, the silanated oxidized samples exhibited a higher activity and a reduced deactivation rate at longer times on stream. Silanation of the HY zeolite without subsequent oxidation allowed almost total suppression of deactivation, especially in the disproportionation of ethylbenzene but also less markedly in the cracking of 3-methylpentane. This suppression of deactivation required avoiding hydrolysis of the SiH group under reaction conditions, and therefore the reactants needed to be absolutely free of traces of water. The maximum deposition of silica that could be reached in one silanation/oxidation step was about 10 wt% at 573 K. Higher temperatures were not possible because of the uncontrolled formation of polysilanes. Simultaneous silanation and oxidation, which means that the oxidant was already present during silanation, allowed controlled deposition degrees higher than 10 wt%. The high-silanated HY samples were found to exhibit shape selectivity in ethylbenzene disproportionation which was not observed on the parent samples. It was shown that pore blocking is the main reason for the decreasing activity with increasing silica deposition in the case of the high-silanated samples. The observed shape selective effects were comparable to those of ZSM-5-type zeolites.