The surface environment and structural evolution of silica supported phosphotungstic acid ( H3PW12O40) catalysts have been investigated as a function of acid loading. H3PW12O40 clusters are deposited intact upon the silica surface, adopting a Stranksi-Krastanov growth mode forming a two- dimensional adlayer which saturates at 45wt% acid. Intimate contact with the silica support perturbs the local chemical environment of three tungstate centres, which become inequivalent with those in the remaining cluster, suggesting an adsorption mode involving three terminal W = O groups. Above the monolayer, H3PW12O40 clusters form three-dimensional crystallites with physico- chemical properties indistinguishable from those in the bulk heteropoly acid. These H3PW12O40/ SiO2 materials are efficient for the solventless isomerisation of alpha- pinene under mild reaction conditions. Activity scales directly with the number of accessible perturbed tungstate sites at the silica interface; these are the active species.