Combined infrared and gravimetric studies have been performed of pyridine adsorption on sulfated and non-sulfated silica-zirconia mixed oxides in order to quantify the site densities of both Bronsted and Lewis acid sites. The values determined for the integrated molar absorption coefficients for the non-sulfated silica-zirconia mixed oxides were 1.24 cm mumol(-1) at 1540 cm(-1) and 1.56 cm mumol(-1) at 1450 cm(-1) and were independent of the Si:Zr ratio and other pre-treatment and preparation variables. On the other hand, values ranging from 0.64 to 1.66 cm mumol(-1) at 1540 cm(-1) for the pyridinium ion and from 1.00 to 2.08 cm mumol(-1) for the Lewis-bound pyridine were determined for the sulfated samples. It is proposed that the reduction in absorption coefficient for the pyridinium ion with increasing sulfate density corresponds with the transformation of monomeric to dimeric or polymeric sulfate species which bring two adjacent adsorbed pyridine molecules into close proximity. The findings of this study raise considerable concern over the validity of acid site densities determined for sulfated materials where calculations have been performed based on literature values for the absorption coefficients.