Vulcan carbon is the favored support for fuel-cell electrocatalysts, but as-received it contains high levels of sulfur (ca. 5000 ppm or greater) which could potentially poison the fuel-cell electrochemistry. The chemical state of the sulfur in platinum/carbon electrocatalysts has been examined by measuring its oxidation state via x-ray photoelectron spectroscopy at different points during the preparation of a "mock" fuel-cell electrode. Also monitored were the presence of sulfate in the aqueous wash from the electrocatalysts and the cyclic voltammetry of the electrocatalysts after each preparation step. Our studies indicate that the platinum catalytically oxidizes some of the covalent sulfur in the vulcan carbon to sulfate when water, heat, and strong physical contact between Pt and C are all present. These conditions are attained during the preparation of typical fuel-cell electrodes. Most of the zero-valent sulfur remains in the carbon after treatment, however, and appears not to be initially in contact with the Pt. This remaining unoxidized sulfur mag be a source of poisoning to the Pt electrocatalyst with long-term electrochemical use, particularly at the fuel-cell cathode.