The influence of the framework Si/Al ratio and the crystal size of HY zeolites on the catalytic alkylation of biphenyl with methanol was studied. A correlation between catalytic activity and the theoretical number of framework Al with no and one Al atom in the second coordination sphere was obtained, indicating that this reaction is catalysed by strong acid sites. The catalytic activity does not change with the crystal size of Y zeolite, therefore, no diffusion limitations for the alkylation reaction on KY catalysts can be expected. However, a lower turnover number for the alkylation reaction was obtained using beta or mordenite zeolites as catalysts, and, diffusion limitations can be expected for these catalysts. Significant differences in the selectivity to para- and ortho-methyl biphenyl products with different framework Si/Al ratios of Y zeolite can be explained supposing an orbital controlled reaction which is governed by the softness of the alkylation complex.