The oxygen evolution reaction (OER) in alkaline solution has been studied for various Ti supported IrO.sTio.,-,, X 2 Pt 0 anodes with 0 5 x 5 0.7 by impedance measurements (EIS) at potentials between -0.6 and l.OV vs. Hg/HgO (NMOE). The electrical parameters of the electrode solution interface were obtained by fitting the experimental impedance data either to a R@, C,)(R,, C,,)L (for the Ir,,,Ti,.,02 electrode) or a Rn(QR,,)L (for PtO,-containing electrodes) equtvalent circuit, where Q = constant phase element. The impedance spectrum in the inert region, -0.6 to 0.45V, is an almost vertical line, indicating that the interfacial impedance is essentially capacitive. The ohmic component obtained from EIS (R, = 2.2Gcm') for the Ir,,,Ti,,, 0 electrode is consistent with the value obtained 2 from the Tafel plot (R, = 2.4Rcm2). The substitution of Pt for Ti in the binary system results in a dramatic increase in the R,, value, ie, the reaction becomes more ditIicult in the presence of this oxide. It was also observed that the introduction of PtO, causes large morphological changes in the binary system. The different behaviour of the IrO, + TiO, system from the other oxide compositions is attributed to the instability of the latter oxides in alkaline solution.