The influence of the Si/Al ratio on catalytic performance of palladium and platinum beta zeolite-based catalysts with or without binder in the hydroisomerization of n-octane was studied. In order to characterize the samples, temperature-programmed desorption of ammonia, surface area measurements, atomic absorption spectroscopy, inductively coupled plasma emission spectrophotometry and chemisorption measurements were used. The catalytic activity of beta zeolite catalysts decreased with increasing Si/Al ratios in samples with or without binder. As expected, a high SUM ratio means a low acid site density. However, the acid strength of the medium-strong acid sites increased with increasing values of this ratio. The isomer selectivity in the palladium catalysts increased with the Si/Al ratio (from 54.3 to 67.8 mol% in samples without binder). The medium-strong acid sites strength of the beta catalysts increased with increasing Si/Al ratios allowing relatively longer residence time of the intermediate carbenium ions on the acid sites. The opposite effect was observed in the platinum catalysts (isomer selectivity shifts from 96.6 towards 78.2 mol% in samples without binder). The presence of hydrogenolysis products in samples with low aluminium content decreased the octane isomer formation. The same effect was observed in the agglomerated catalysts but not as pronounced as in the non-agglomerated ones. (c) 2005 Elsevier B.V. All rights reserved.