The main advantage of oxidative dehydrogenation (ODH) of light alkanes over non-oxidative dehydrogenation (NODH) is the absence of thermodynamic limitations for olefin per-pass yield. It is shown, however, that considerable amounts of molecular hydrogen are formed (in some cases-equivalent to the amounts of produced olefin or even higher) over typical ODH oxide catalysts when C-2-C-4 alkanes and oxygen are present in the feed gas. The analysis of product yields as functions of reaction parameters (temperature, flow rate, hydrocarbon-to-oxygen ratio) over a series of V- and Sb-containing ODH catalysts has shown that the main kinetic features cannot be described in the framework of a single mechanism. NODH, coking and free-radical oxidation are considered as the most important H-2 formation pathways. The ways to optimize the olefin production depending on the predominant mechanism of hydrogen formation over a particular catalyst are discussed. (C) 2003 Elsevier Science B.V. All rights reserved.