The acid sites of a series of phosphated silica - zirconia catalysts have been quantified by the combined use of FTIR and TGA and the data related to the activity of the samples for the isomerisation of but-1-ene. The catalyst series show a linear correlation indicative of constant TOF when the number of Bronsted acid sites is plotted against catalytic activity. However, this relationship only holds when the number of Lewis acid sites exceeds that of Bronsted acid sites suggesting that an optimal arrangement may involve both types of site. The TOF is less when the Lewis/Bronsted ratio falls below 1, a scenario that dominates at high Bronsted acid site densities. When this site arrangement prevails, a higher than expected cis/trans ratio is found in the isomerisation selectivity. These differences in the environment of the active Bronsted acid site may also be deduced from differences in the molar absorption coefficients of adsorbed basic probe molecules.