For atmospheric conditions a hydrocarbon undergoes dehydrocyclization by a mechanism that involves nearly irreversible adsorption of the reactant. Hence, there is essentially no H/D exchange when a mixture of D, and hydrocarbon are passed over the catalyst. The current study shows that this is not the case for conversions at higher pressures (6.8 atm; 0.69 MPa). At the higher pressure, there is exchange of H and D to produce a nearly statistical distribution of H and D in the unconverted reactant as well as the products. The pressure dependence is consistent with a mechanism in which the H-2(D-2) pressure limits the second dehydrogenation step. This mechanism accounts for the H2 dependence of both H/D exchange and the competitive conversion of a mixture of alkane and cycloalkane. (c) 2005 Published by Elsevier B.V.