Isomerization of light alkanes and alkenes was investigated on pure MoO3 powder which was gradually reduced under a rapid flow of hydrogen (1 atm) at 350degreesC. Incompletely reduced MoO3 samples readily isomerize alkanes in the order nC(4) much less than nC(5) < nC(6) = nC(7) = nC(8). The rates are at least as high as those measured on platinum metal. Isomerization of oletins is always faster than those of the corresponding alkanes. As a consequence of the inhibiting effect of water vapor on the reduction of the molybdenum oxides, very small MoO3 samples exhibit short-lived isomerization properties whereas larger samples whose reduction gives rise to higher water vapor level are more longer active. With all hydrocarbons marked poisoning effects by hydrocarbonaceous deposits take place. Physical adsorption of nitrogen and argon at - 195 degreesC shows that the samples become microporous on reduction. X-ray analysis of reactive samples always reveal the presence of two molybdenum oxides, i.e. MoO2 and an up-to-now undetermined oxide MoOx which is known to form only at low temperatures; in addition large amounts of amorphous materials are always present. XPS analysis and the catalytic results suggest that the suboxides containing only Mo6+ and Mo5+ ions are not responsible for the isomerization; then the formation of the isomerizing sites results from a deeper reduction state of the MoO3 sample. Many features of the reaction are in favor of a mechanism of the bifunctional acid type.