The activity of the electrochemical water oxidation catalyst based on Ru-red incorporated in an electrodecoated Nafion membrane was studied. The complex worked as an active catalyst, and an optimum concentration for the catalyst turnover number (TN) was exhibited. The TN increased with the concentration in low concentrations because of facilitated charge transfer between the catalysts, and the increase in the concentration brought about decreased TN due to bimolecular decomposition of the catalyst. An activity model for the electrochemical catalyst in a membrane was proposed based on intermolecular distance distribution, and the activity was analyzed in terms of a charge transfer distance (r(0)/nm) and a critical decomposition distance (r(d)/nm). Their values were obtained as 1.25 and 1.21 nm, respectively.