We report temperature-dependent characterization of the electrical and optical properties of GaAs single junction solar cells up to 450 °C. An external quantum efficiency (EQE) of 75% was maintained at temperatures up to 300 °C, with a corresponding increase in the absorption edge as a function of wavelength due to the decrease in band gap with temperature, in agreement with theory. Above 300 °C, the EQE decreased strongly as the temperature was further increased. This drop in EQE resulted in a corresponding decrease in short-circuit current, also observed in the I-V characteristics as a function of temperature. When cooled back to room temperature the short-circuit current nearly fully recovers whereas the open-circuit voltage is found to be irreversibly degraded. The origin of the degradation is discussed. Modeling is used to support the experimental results.