The catalytic properties of MoO3 with and without Pd for the isomerization of heptane have been investigated at 523 K under atmospheric pressure. The isomerization activity of Pd/MoO3 increased with the period of H-2. reduction at 623 K, and became almost constant after 36 h reduction. This behavior was almost the same as that of MoO3, although the reducibility of MoO3 was markedly improved in the presence of Pd. The reduced Pd/MoO3 was al,out 2.5 times as active as the MoO3. The mixture of MoO3 and Pd/SiO2 exhibited a similar catalytic performance to Pd/MoO3, indicating the important role of hydrogen spillover. H-2 reduction was found to increase the surface area of MoO3, especially in the presence of Pd. The isomerization activity of reduced Pd/MoO3 was dependent on the amount of Pd loading up to 0.01 mol%. We conclude from these results that the high isomerization activity of Pd/MoO3 can be attributed to its large surface area as well as to the promotion of dehydrogenation-hydrogenation steps by Pd.