The contribution to the electrical conductivity of grain boundaries in polycrystalline Cu2O was measured in vacuum and in air between 200° and −145°C. It was found to follow a simple exponential temperature dependence with a single activation energy of about 0.31 eV. Heat treatment at 200°C can be used to vary the contribution of the grain boundaries from 5×10−9&OHgr;−1in air to 8×10−10&OHgr;−1in vacuum at room temperature. The effects are explained by impurities conglomerating at the grain boundaries and the diffusion of oxygen along these boundaries. The deterioration of the volt‐ampere characteristics of cuprous oxide rectifiers after preparation or as a result of extensive heating above 150°C is explained by the same mechanism. The studies of the rectifiers were based on measurements on capacity and resistance of the barrier layers at room temperature.