In the presence of water vapor and gaseous oxygen, lower olefins such as n-butenes are subject to oxidative scission over V 2O 5MoO 3 catalysts at 170–250 °C to form carboxylic acids and/or aldehydes. To examine the mechanism of this reaction, the effects of the acid-base character of catalysts and the feed of water vapor and gaseous oxygen on the reaction were investigated. The results show that, while acidic sites of catalysts, water vapor, and gaseous oxygen are all necessary for the reaction, water vapor in particular plays an important role, the cut of its feed causing a total loss of reactivity of olefins at temperatures below 250 °C. Results also show that the reaction proceeds consecutively through the oxyhydrative scission mechanism previously proposed; an olefin is hydrated on an acidic site of the catalyst to form an alcohol followed by oxidation to a ketone and oxidative scission to the final products. The oxidation of C 5 ketone isomers indicates that the last step, the oxidative scission of ketones, follows Popoff's law. In addition, reactivity orders of assumed intermediates show that, of the multiple reaction steps, the oxidative scission step is rate determining in propylene oxidation, while this step is faster than others in butene oxidation.