The exchange of hydrogen for deuterium (H/D exchange) in cyclopentane (CP) is a promising test reaction for studying support effects in heterogeneously metal-catalyzed reactions, such as differences in stability and activity in, e.g., benzene or tetralin hydrogenation reactions. In order to employ this test reaction, a full understanding of the H/D exchange mechanism is essential. However, clear insight in the observed kinetics and selectivities is lacking in the literature. In this paper, a kinetic model that adequately describes the observed activity and orders in CP and D-2 is developed. It is shown that the selectivity is determined in a series of reactions after the rate-determining step. To understand the observed selectivities a Monte Carlo model is developed which accurately simulates the observed exchange patterns and reveals the relative contributions of four competitive intermediates. One intermediate (a pi-bonded eta(2)-cyclopentene) can rotate in a nonactivated mechanism via an allylic intermediate. The model reveals the number of rotations each intermediate experiences and can be as high as similar to20, even if only 5 D atoms are observed in the product. This number of rotations is a better measure of the H/D exchange than the conventional "multiplicity." The H/D exchange of CP together with these insights can now be applied to investigate in detail the influence of the support on the catalytic properties of supported metal particles. (C) 2003 Elsevier Science (USA). All rights reserved.