Mudcrack-free oxide films of IrxTi1-xO(2) (0 < x less than or equal to 1) on titanium substrates were obtained, and the effects of the oxide composition on the rate of oxygen-evolution reaction were investigated. At x greater than or equal to 0.6, Ir-rich grains appear on the mudcrack-free surface. In the purely single-phase region (0 < x less than or equal to 0.5), the pseudo-capacitive charge is proportional to the surface composition, x(s), and the exchange-current density for the oxygen-evolution reaction increases linearly with x(s) at 0.2 less than or equal to x(s) less than or equal to 0.5, with an extrapolated intercept at x(s) similar to 0.15, below which the oxides are inactive.