The electrical conductivity of high‐purity single‐crystal alumina is determined in a temperature range from 400° to 1300°C. By applying an advanced fully guarded threeterminal measurement technique, reliable conductivity measurements are performed to as low as 10−17Ω−1· cm−1. Gas and surface conduction are measured separately and shown to be negligible. High‐purity sapphire exhibits a conductivity of 10−16Ω−1· cm−1at 400°C, two characteristic activation energies of 0.4 and 4.8 eV with increasing temperature, and a conductivity of 3 × 10−8Ω−1· cm−1at 1300°C. The fraction of the current carried by ions is determined by electron probe analysis of the electrodes following a 640‐h transference test at 1200°C with 4 kV/cm field applied. Only 0.3% of the current at 1200°C is carried by ions. A mathematical model of electrical conduction in sapphire is developed which describes sapphire as a wide‐bandgap semiconductor, doped with one dominant donor and one dominant acceptor. The observed conductivity is well described by the model over the enti