Electrocatalytic H-2 formation was carried out by using a Nation membrane (Nf) embedding a Pt catalyst as well as an alkyl viologen (denoted as RV2+; alkyl group (R): methyl, ethyl, propyl, heptyl, or dodecyl group). The H+ reduction catalyzed by the Pt via the electrogenerated RV.+ was investigated. In order to understand the electron-mediating characteristic of the RV2+ on kinetic aspect, turnover numbers (TON) of the RV2+ for H-2 formation as well as electron transport kinetics (represented by the apparent diffusion coefficient of electron, D-app) of the mediator were studied with respect to RV2+ concentration. In any RV2+ system, both the TON and the D-app exhibited a similar dependence on the concentration, showing that the overall kinetics is dominated by the electroreduction process of the mediator. When applying both the tetrachloroplatinate(II) (Pt-0 in catalytically active state) and the viologen mediator to the H-2 formation system, the TON was found to be controlled by the D-app. (C) 2003 Elsevier Science B.V. All rights reserved.