We have investigated the kinetics of ligand rebinding after nano-second laser photolysis of horse heart carboxymyoglobin, encapsulated in wet silica gels. The geminate ligand rebinding to the gel embedded protein shows a markedly enhanced efficiency, 0.43, with respect to solution. This geminate phase can be described by a stretched exponential decay. The second-order rate of CO rebinding to myoglobin (Mb) gels is 1.42 x 10(6) M-1 s(-1). The bimolecular binding rate is thermally activated with barriers of approximately 4 kcal mol(-1). These findings indicate that Mb within silica gels is in an environment with an apparent higher local viscosity than in solution.