Titanium is one of the most stable materials, remaining undissolved under strongly acidic polymer electrolyte fuel cell (PEFC) conditions. Nanocrystalline Pt-Ti alloy electrocatalysts with a catalyst grain size of several nanometers have been successfully prepared by the colloidal and sol-gel method, characterized by X-ray diffraction, field-emission scanning electron microscope, scanning transmission electron microscope, energy-dispersive X-ray analysis, and X-ray photoelectron spectroscopy. Electrochemical activities were evaluated by current-voltage characteristics measurements using membrane electrode assemblies for humidified H-2 fuels with or without 100 ppm CO, cyclic voltammetry, and CO-stripping voltammetry in 0.1 M HClO4. Pt-Ti alloy electrocatalysts exhibited higher electrochemical performance than pure Pt catalyst, i.e., higher activity for oxygen reduction reactions and improved CO tolerance. (C) 2007 The Electrochemical Society.