This paper describes the effects of varying the Pt to Ru ratio in carbon-supported catalysts for methanol oxidation as a function of temperature. Previously these effects were studied in isolation, but now it is shown that the composition of a given catalyst as a function of temperature is extremely important for its activity towards methanol oxidation. Platinum rich 3:2 atomic ratio catalysts perform better than a 1:1 catalyst at 25 degreesC, where only Pt is believed to be active towards methanol dehydrogenation, since this process is a highly thermally activated process on Ru sites. This result is reversed at 65 degreesC, where the 1:1 catalyst displays much higher currents across the entire range of polarization. This may result from methanol dehydrogenation occurring on both Ru and Pt sites at higher temperatures. At an intermediate temperature, 45 degreesC, the 3:2 catalyst is seen to perform better at lower current values, while the 1:1 catalyst is superior at higher current densities, with the crossover occurring at 62 A g(-1). As a consequence, when designing fuel cell catalysts, the composition of the catalyst employed should be tailored with respect to the exact operating conditions, in order to promote optimum fuel cell performance.