The conversion of 2-methylpent-2-ene (2M2PE) is employed as a probe reaction to investigate the acidic properties of 12-tungstophosphoric acid for various loadings on silica, residence times, time-on-stream and reaction temperatures. The high selectivities to methyl shift products as compared with those to double-bond shifts indicate the presence of strong acid sites. Infrared spectra show that adsorbed 2M2PE exists as saturated species on the surface of HPW/SiO2 with irreversibly adsorbed material forming conventional coke. Similar conversions of 2M2PE and selectivities to products are obtained with 12-tungstosilicic acid supported on SiO2 while with 12-molybdophosphoric acid the conversions are markedly lower and the highest selectivities are found with products obtained from double-bond shifts, indicating the presence of relatively weak acidic sites. The results obtained from 2M2PE conversion on other acidic catalysts are compared with those from the supported heteropoly acids.