The durability of Pt-Co alloy cathode catalysts supported on high surface area carbon is investigated by subjecting them to accelerated stress tests (ASTs). The catalysts had different initial Co contents and nanoparticle morphologies: a “spongy” porous morphology for the high-Co (H) content catalyst, and a fully alloyed crystalline morphology for the medium-Co (M) and low-Co (L) content catalysts. The specific activity of the catalysts depends on their initial Co content, morphology and nanoparticle size, and remained higher than 1000 μA/cm²-Pt after 27–50% Co loss. The H-catalyst electrode showed the smallest kinetic overpotentials (η^c_s) due to higher initial Pt loading than the other two electrodes, but it had the fastest increase in η^c_s with AST cycling due to lower Co retention; the L-catalyst electrode showed higher η^c_s due to a lower initial Pt loading, but had a smaller increase in η^c_s with aging due to higher Co retention; the M-catalyst electrode showed a similar increase in η^c_s with aging, but this increase was due to the combined effects of Co dissolution and electrochemically active surface area (ECSA) loss. The modeled increase in mass transfer overpotentials with aging correlates with the initial Pt loading, ECSA loss and the initial catalyst morphology.