The corrosion of copper in simulated deep repository conditions has been studied using thermodynamic calculations and short-term corrosion rate measurements. Thermodynamic calculations have been used to predict the corrosion behavior of copper in 0.0015, 0.4, and 1.5 m chloride solutions at 80°C. The results of the calculations were summarized in Pourbaix diagrams. A combination of experimental techniques (weight loss, solution analysis, and electrochemical impedance spectroscopy) was used to investigate the extent of corrosion in simulated ground waters with different salinity. The results were found to be in accordance with thermodynamic predictions. The corrosion rates in simulated saline ground waters were significant, whereas they were at the limit of detection in simulated fresh groundwater. The corrosion rates have been shown to correlate well with the levels of residual oxygen in the solutions. This means that studies in conditions as close as possible to anoxic are required in order to estimate the possibility of copper corrosion in simulated deep repository conditions. In addition, corrosion current densities determined from impedance spectroscopy were several times too high compared with weight loss and solution analysis data, indicating the inaccessibility of the true polarization resistance from the impedance data.