The rate of exchange of deuterium between hydrogen gas and water catalyzed by pentacyanocobaltate(II) has been measured as a function of pressure and temperature at a constant total cobalt concentration, 0.05 M, and ionic strength, 1 M. The rates of evolution of hydrogen from solutions of hydridopentacyanocobaltate(III) have been determined under identical conditions. These data have been employed to test the validity of a mechanism for the-catalyzed isotope exchange reaction, and to clarify the controversy over the magnitude of the specific rate for the deuterium transfer stepCalculations show that kinetic isotope and ion pairing effects are significant.