Rebinding kinetics of CO to microperoxidase-8 (Mp), an excellent model system for the active site of heme proteins such as myoglobin and hemoglobin, was measured after photolysis of MpCO in solutions with various viscosities and temperatures, using femtosecond vibrational spectroscopy. Whereas the geminate rebinding of CO to Mp in water is negligible, significant fractions of CO rebind nonexponentially within 1 ns at room temperature in a glycerol/water solution. The geminate yield of the CO rebinding increases and its rate accelerates as the viscosity of the solution increases either by increasing glycerol content in glycerol/water mixtures at 294 K or by decreasing temperature of the solution from 323 to 283 K. The nonexponential rebinding kinetics can be described by the theory of a diffusion-controlled reaction and the data are well reproduced by the pair survival probability function in the absence of any interaction potential between the pair. The rebinding kinetics was also successfully described by the SRC model, a distributed linear coupling model for the CO rebinding.