The replacement reaction of myoglobin (Mb), MbCO + O2 leads to MbO2 + CO leads to MbCO + O2, has been studied with flash photolysis in the temperature range from 140 to 320 K and the time range from 2 mus to 200 s. In a fraction of the Mb, the photodissociated CO remains within the protein; rebinding is not affected by the presence of O2 and occurs with rates that are identical with the ones observed earlier in solvents containing only CO. In the remaining fraction CO migrates into the solvent and Mb combines preferentially with oxygen. The rate of the subsequent replacement of O2 by CO permits calculation of the oxygen dissociation rate ko2; ko2 has been determined from 260 to 320 K. The measurements support a multibarrier model.