The surface area of H₂-reduced 0.01 mol.% Pt/MoO₃ increased in proportion to the extent of reduction, and reached the maximum value of 250 m² g⁻¹ at a reduction degree of 60–70%. In the case of H₂-reduced MoO₃, the relationship between the surface area and the reduction degree was dependent on reduction temperature. MoO₃ reduced at 673 K exhibited a much smaller surface area than that at 623 K even when the reduction degree was comparable. XRD studies showed that reduction of Pt/MoO₃ proceeded ia the formation of H_xMoO₃ phase, irrespective of reduction temperature. In the reduction of MoO₃ without Pt, however, the H_xMoO₃ phase was formed at temperature below 623 K. The heptane isomerization activity of H₂-reduced MoO₃ and Pt/MoO₃ was similarly dependent on the reduction degree as the surface area. H₂-reduced MoO₃ and Pt/MoO₃ catalyzed the dehydrogenation and dehydration of propan-2-ol simultaneously, indicating the presence of dual sites. There were good relationships between the isomerization activity and the dehydration activity. We suggest from these results that the surface area and the catalytic activities can be enlarged when reduction of MoO₃ proceeds through the formation of a H_xMoO₃ phase.