The effect of pressure on proton binding to the racemic isomer of the cobalt(l) macrocycle, CoL+ (L = 5,7,7,12,14,14-hexamethyl-1,4,8,1 1-tetraazacyclotetradeca-4,11-diene), has been studied for a series of proton donors using pulse radiolysis techniques. The second-order rate constants for the reaction of CoL+ with proton donors decrease with increasing pK(a) of the donor acid, consistent with a reaction occurring via proton transfer. Whereas the corresponding volumes of activation (DeltaV(double dagger)) are rather small and negative for all acids (proton donors) with pK(a) values below 8.5, significantly larger negative activation volumes are found for weaker acids (pK(a) > 9.5) containing OH groups as proton donors. In the latter case, the observed DeltaV(double dagger) for these protonation reactions show a correlation with the reaction volumes (DeltaVdegrees(ion)) for the ionization of the weak acids with a slope of 0.44, indicating that bond dissociation of the weak acid molecule bound to the metal center proceeds approximately halfway at the transition state along the reaction coordinate in terms of volume changes.