The influence of platinum on the corrosion of carbon catalyst supports has been characterized by on-line mass spectroscopy during cyclic voltammetry, with varying Pt mass fraction, catalyst type, and temperature. The CO2 generation rates increased with higher Pt mass fraction (0, 10, and 39% balanced by Vulcan XC-72). A peak observed at approximately +0.55 VRHE for Pt/C was lowered to +0.45 V-RHE for PtRu/C. An Arrenhius plot indicated higher apparent activation energy for CO2 production at the positive potential limit of the cyclic voltammogram on 0% Pt (carbon-only) electrode than on 39% Pt/C electrode. It was concluded that platinum accelerated the corrosion rate of the carbon catalyst support. (C) 2003 The Electrochemical Society.