The activity of an electrocatalytic oxygen evolving center (OEC) molecule based on the trinuclear Ru complex, Ru-red ([(NH3)(5)Ru-O-Ru(NH3)(4)-O-Ru(NH3)(5)](6+)) codispersed with an amino acid residue model compound in a Nafion membrane was studied. The activity of the OEC molecule increased remarkably in the presence of p-cresol (p-Cre), a Tyr model compound. The activity was analyzed based on the intermolecular distance distribution in terms of charge transfer distances between the OEC molecules mediated by p-Cre, their critical decomposition distance, and the intrinsic activity. The charge transfer distance increased remarkably from 1.28 to 2.25 nm by the presence of p-Cre, suggesting that p-Cre works as a mediator for the charge transfer.