Electrochemical and microstructural measurements of membrane electrode assemblies (MEAs) cycled under cold-start conditions are reported. An experimental protocol using a single-cell fixture was developed for MEA durability tests under cold-start cycling. Electrochemical diagnostics using high-frequency resistance and pure O-2 found that MEA no. 1 cycled under 100 mA/cm(2) from -30 degrees C does not show any degradation after 100 cycles, MEA no. 2 cycled under 300 mA/cm(2) from -30 degrees C exhibits mild degradation after 150 cycles, and MEA no. 3 cycled under 500 mA/cm(2) from -20 degrees C suffers severe degradation after 110 cycles. Transmission electron microscopy and X-ray diffraction using cross-sectional samples of the aged MEAs further revealed three primary degradation mechanisms: (i) interfacial delamination between the cathode catalyst layer (CL) and membrane, (ii) cathode CL pore collapse and densification upon melting of a fully ice-filled CL, and (iii) Pt particle coarsening and Pt dissolution in perfluorosulfonic acid ionomer. The interfacial delamination and CL densification appear to be closely related to each other, and the key parameter to affect both is the ice volume fraction in the cathode CL after each cold-start step. Eliminating or minimizing these two degradation processes could improve the MEA cold-start durability by 280%. Mitigation strategies, such as improved gas purge prior to cold start, better MEA design, low startup current density, and low cell thermal mass, are proposed. (c) 2008 The Electrochemical Society.