Molybdenum trioxide samples having apparent particle sizes (APS) of 5 and 20 mu m were partially reduced under flow of a mixture of H-2/n-heptane during 4 h. X-ray diffraction and Raman spectroscopy showed the typical structural transformation of MoO3 into MoOxCy and MoO2. These structural changes occur preferentially on the{0k0} planes. After the reduction treatment the resulting materials, having surface areas of 23 and 53 m(2)/g, were evaluated in the isomerization of n-heptane at 643 K and 18.5 bar. The catalyst with an APS of 20 l. m showed a maximum conversion around 70%, while for the catalyst with an APS of 5 mu m the maximum conversion was 34%. The lower activity of the 5 l. m MoOxCy catalyst seems to be related to a faster rate of formation of oxygen vacancies and rearrangement of the lattice into a more stable and less active structure in the case of small-size particles, due to a higher concentration of terminal Mo=O bonds along the a- and b-axes, which facilitate the electrophilic attack by hydrogen on the ( 010) plane.