For various oxidoreductases, the optimum pHs of the enzymes can be calculated using the rule based on proton transfer. Relative probability of a certain amino acid side chain to be in the water, or the relative affinity to the water was calculated using Boltzman distribution. Also, the protonated and deprotonated portions of a certain amino acid side chain were calculated using p$K_R$ of that and the effective protonated and deprotonated protions were the product of relative probability and the protonated and deproteonated protions. Where the total effective protonated portion was equal to the effective deprotonated portion of amino acid side chains, it was expected that oxidoreductases have max-imum activities. The optimum pHs calculated by our rule were compared with the experimental results.