A quantitative analysis of catalyst layer degradation was performed with X-ray photoelectron spectroscopy (XPS). XPS is quantitative, surface-sensitive, and is able to distinguish different bonding environments or chemical states of fuel cell catalyst layers and polymer electrolyte membrane. These capabilities have allowed us to explore the complex mechanisms of degradation during fuel cell operation. The elemental surface concentrations of carbon, fluorine, oxygen, sulfur, and platinum on the catalyst layer surface were measured before and after fuel cell operation, and the different chemical states of carbon and platinum were identified. Both XPS analysis and scanning electron microscopy revealed that the ionomer on the catalyst layer degraded or decreased in concentration after fuel cell operation. Ionomer degradation was characterized by a decrease of CF3 and CF2 species and an increase in oxidized forms of carbon (e.g. C-O and C=O) and an increase in less- and non-fluorinated forms of carbon (e.g. CF and graphitic), consistent with overall reduction of fluorine by about 22%. The surface concentration of fluorine and platinum also reduced from 50.1% to 38.9% and from 0.4% to 0.3%, respectively. The concentration of oxidized forms of carbon and platinum increased after fuel cell operation. The surface-sensitive XPS technique should prove useful for the quantitative monitoring of catalyst layer degradation mechanisms over the lifetime of fuel cells. Published by Elsevier Ltd.