Excellent catalytic activity was observed for the electroreduction of H2O2 by a nontronite (SWa-1, ferruginous smectite) clay coating on a glassy carbon electrode (GCE) with incorporated methyl viologen (MV(2+)) as mediator. The effect of pH on the electrochemistry of MV(2+) and Ru(NH3)(6)(3+) exchange cations in montmorillonite- and nontronite-modified GCEs provides insight into the electrochemical accessibility of redox active species in the electrodes. We find that the number of surface-bound MV(2+) ion pairs capable of accessing the electrode surface decreases with increasing acidity. In contrast, the electroactivity of Ru(NH3)(6)(3+) exchange ions intercalated in the gallery region of the clay film is pH independent. The measured value of the rate of concentration of the electroactive species to that of the total cation absorbed for the nontronite-modified GCE is about 4.3 times higher than that of the montmorillonite-modified GCE. These results, together with the difference in the octahedral and tetrahedral charge distributions between montmorillonite and nontronite, lead to the following conclusion. The active site for H2O2 reduction is iron on the tetrahedral edge surfaces of nontronite, and mainly ion-paired species of MV(2+) adsorbed in excess of the cation exchange capacity onto the tetrahedral edge surfaces contribute to the excellent catalytic activity.