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All isomeric alkanes from methane to the hexanes, cyclopentane, and cyclohexane, undergo hydrogen-deuterium exchange with a deuterium oxide-acetic [2H1] acid solvent catalysed by potassium tetrachloroplatinate( 11). The catalyst is stabilised by the acidity of the solution and the presence of a small concentration of a weak complexing agent such as pyrene. Exchange is preceded by dissociation of chloride ligands from the added tetrachloroplatinate(l1) to form an uncharged platinum(l1) complex which may be monomeric or dimeric. The relative reactivity for the hydrogen-deuterium exchange is in the order primary > secondary > tertiary which is in the reverse order to the reactivity expected from bond energy data. Exchange rates in the series of n-alkanes have been related to their ionisation potentials. Branched chain alkanes are less reactive than their respective n-isomers, whereas the cycloalkanes are more reactive. The results indicate that the initial interaction between the alkanes and the platinum involves a reversible electron transfer from the delocalised molecular orbitals in the alkanes to the uncharged platinum(l1) complex. Subsequent steps in the proposed exchange mechanism involve a reversible oxidative-addition of the alkane to the platinum(i1) complex to form a hydrido-platinum(1v) complex which can lose HCI to, and gain DCI from, the solvent.