Skeletal isomerization of n-hexane and 1-hexene was investigated on molybdenum dioxide MoO2 powder which was gradually reduced under a rapid flow of hydrogen (1 atm) at 350degreesC. Significant isomerization was observed with the commercially available MoO2 samples whose overall behavior was qualitatively apparented to the one previously reported of the trioxide MoO3, but not quantitatively. On the other hand, the same experiments conducted with MoO2 prepared in the laboratory showed always negligible isomerization but mainly cracking reactions. The differences between the two sorts of molybdenum dioxide were attributed to the presence in the commercial dioxide of a few percents of Mo6+ oxide-as confirmed by DRX and XPS analysis-whose reduction products are well known to act as very efficient skeletal isomerizing catalysts. Hence, only the results obtained with the purer laboratory made MoO2 are considered to reflect the real properties of this compound. As a consequence, the skeletal isomerizing properties of partially reduced MoO3 powder can hardly be attributed to the molybdenum dioxide MoO2-associated or not with molybdenum metal-which, among other products, is always present in the resulting material.