The conversion of isobutane and the selectivities to products are studied on silica-supported 12-tungstophosphoric acid (H3PW12O40, abbreviated as HPW) and 12-molybdophosphoric acid (H3PMo12O40, abbreviated as HPMo) for various loadings, reaction temperatures, contact times and times-on-stream. The conversions and selectivity patterns are markedly dissimilar for the two acids with carbenium ion and hydride transfer processes catalyzed on the former at temperatures as low as 200 degreesC but not on the latter, a reflection of the stronger acidic sites on the tungsten-containing acid than on 12-molybdophosphoric acid.