Reaction rate constants have been acquired for the gaseous unimolecular decomposition reaction of the Co+(OC(CH3)(2)) cluster ion and its deuterium labeled analog. Each rate constant is measured at a well resolved cluster internal energy within the range 12 300-16 100 cm(-1). The weighted, averaged kinetic isotope effect (KIE), k(H)/k(D) = 1.54 +/- 0.05, is about three times smaller than the KIE measured for the rate-determining rate constants in the similar Ni+(OC(CH3)(2)) decomposition reaction. These reactions likely follow the same oxidative addition-reductive elimination mechanism. Thus, this unexpected change in the KIE magnitudes is not due to differences in the dissociative reaction coordinates. Rather, we propose that the unique dissociation dynamics of these two similar systems is due to differences in the low-lying electronic structure of each transition metal ion.