The heptane isomerization activity of Pt/MoO 3–SiO 2 reduced by H 2 at 773 K for 6 h was investigated, in particular its relation to the physicochemical properties. H 2-reduced Pt/MoO 3–SiO 2 with a larger amount of MoO 3 had a higher isomerization activity. The same tendency was observed in the activity for 2-propanol dehydration. The isomerization activity was related to the number of acid sites, which was determined from NH 3-TPD. XRD studies of H 2-reduced Pt/MoO 3–SiO 2 showed that the formation of MoO x H y was promoted as the MoO 3 loading was increased. The conversion of MoO 3 to H 1.68MoO 3 in the initial stage of reduction varied with the amount of MoO 3; only a part of MoO 3 was converted to H 1.68MoO 3 in Pt/MoO 3–SiO 2 with a small amount of MoO 3, while the complete conversion to H 1.68MoO 3 was observed in Pt/MoO 3. Based on these findings, we conclude that the isomerization activity of H 2-reduced Pt/MoO 3–SiO 2 can be governed by the formation of MoO x H y , which is yielded from the reduction of MoO 3 through H 1.68MoO 3.