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The effect of pressure on the formation and deoxygenation kinetics of oxymyoglobin was studied by using temperature-jump and stopped-flow techniques, respectively. The corresponding volumes of activation are +5.2 ± 0.5 and +23.3 ± 1.8 cm3 mol'1, which result in a reaction volume of -18.1 ± 2.3 cm3 mol'1 for the system Mb + 02 == Mb02. The latter was also measured directly from the pressure dependence of the equilibrium constant and resulted in a reaction volume of -19.3 ± 1.5 cm3 mol'1. A volume profile analysis indicated that bond breakage during deoxygenation proceeds according to a dissociative mechanism with almost complete Mb-G2 bond cleavage in the transition state. This suggests that almost no bond formation with the heme center occurs in the transition state during the oxygenation of myoglobin. The volume increase

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