Mixtures of molybdenum metal and suboxides. resulting from a partial reduction of MoO3 by hydrogen or by a mixture of hydrogen and alkane in the range 300-400 degreesC, are known for their ability to readily isomerize alkanes in the presence of hydrogen. In this work we found that this property is transitive when very high space velocities values of the gaseous flow are attained, cracking of the alkane becoming the main reaction after a few hours. This occurred because in the presence of the reducing flow, molybdenum oxides - and in particular the previously proposed isomerizing species as molybdenum dioxide and molybdenum oxycarbide - tend inevitably to the metallic state. The opposite behaviour reported in the literature - high and stable isomerization activity with time -results only from the molydenum oxides reduction rate slow down, owing to a strong inhibition by the water vapour. This banal auto-inhibition effect - but so far ignored in the isomerization studies becomes predominant when the space velocity value of the gaseous flow is too low.