Oxygen-depolarized cathodes consisting of gas-diffusion electrodes (GDEs) for electrolysis in a chlor-alkali cell at 90 C were studied. The electrode design was based on a carbon-free catalyst and comprised of a mixture of micronized silver particles, a small amount of Hg and PTFE binder. The cathodes were investigated by electrochemical measurements, and surface and morphological analyses before and after different operation times in chlor-alkali cells. Electrode stability was investigated by life-time tests. The surface properties of gas diffusion electrodes were studied for both fresh and used cathodes by X-ray photoelectron spectroscopy (XPS). Transmission (TEM) and scanning electron microscopy (SEM-EDX) were used to investigate morphology. The bulk of gas diffusion electrodes was studied by X-ray diffraction (XRD) and thermogravimetric analysis (TG-DSC). At least two main degradation processes that occur on different time-scales were identified and attributed to segregation and loss of the second metal at the interface and a decrease in the hydrophilic properties with time. Furthermore, an increase in the precipitation of compounds from the reaction process also decreased performance by the occlusion of reaction pores.