The degradation of the superconducting properties of REBa2Cu3O7–x coated conductors when heated above 150 °C is a topic of concern for any REBa2Cu3O7–x-based applications. This study makes it clear the role of thermally activated oxygen out-diffusion processes in the modification of the superconducting properties of commercial coated conductors heat treated in air. We point out that oxygen can diffuse out of the REBa2Cu3O7–x layer along two main channels: one associated with the grain boundaries and a second one associated with the superconducting grains. The activation energy is lower for the oxygen out-diffusion events that take place at the grain boundaries, and this is the main cause of the degradation of the critical current at temperatures lower than ∼250 °C. More specifically, through a systematic investigation of the electrical and magnetic properties, we demonstrate that the decrease of the critical current upon heating the coated conductor is initially due to a loss in the transparency of the REBa2Cu3O7–x grain boundaries, which determines a reduction of the effective superconductor cross section available for the superconducting current. The degradation of the intragrain properties becomes important only above ∼250 °C and manifests itself through a simultaneous variation of the critical current and of the critical temperature.