The increase in the rate of the primary back reaction on cooling the photoreaction center from Rhodospirillum rubrum was interpreted in terms of a model in which the peculiar temperature dependence of the rate results from a phase transition involving water. The primary back reaction is defined as the return of the electron from the reduced primary ubiquinone to the oxidized bacteriochlorophyll molecules following illumination. The dye 8-anilino-1-naphthalene sulfonate was used to detect the state of the water solvent as it transforms on cooling from a liquid to a solid glass. We inferred from studies with air-dried films of photoreaction center that the water which may be responsible for the unusual temperature dependence of the rate of the primary back reaction is not on the surface but is bound within the photoreaction center protein.