A family of mesoporous SBA-15 supported H3PW12O40 (HPW) catalysts were synthesized by wet impregnation and compared with fumed silica analogues for the solventless isomerization of alpha-pinene under mild conditions. Structural and acidic properties of supported HPW materials were characterized by powder XRD, HRTEM, XPS, TGA, N-2 porosimetry, DRIFTS, and ammonia and propylamine chemisorption and TPD. The high area, mesoporous SBA-15 architecture facilitates the formation of highly dispersed (isolated or low dimensional) HPW clusters and concomitant high acid site densities (up to 0.54 mmol g(-1)) relative to fumed silica wherein large HPW crystallites are formed even at low HPW loadings. alpha-Pinene exhibits a volcano dependence on HPW loading over the SBA-15 support due to competition between the number and accessibility of acid sites to the non-polar reactant, with the superior acid site accessibility for HPW/SBA-15 conferring a 10-fold rate enhancement with respect to HPW/fumed silica and pure HPW. Monocyclic limonene and terpinolene products are favoured over polycyclic camphene and beta-pinene by weaker polyoxometallate analogues over SBA-15. (C) 2016 The Authors. Published by Elsevier B.V.