The room‐temperature electrical properties of 28 semi‐insulating GaAs crystals have been determined by using a mixed‐conductivity analysis. It is shown that for most of these samples, such an analysis gives good accuracy for the electron mobility μn and electron concentration n, but poorer accuracy for the hole mobility μp, hole concentration p, and intrinsic concentration ni. The intrinsic concentration is determined at 296 °K to be ni ≈ (1.7±0.4) ×106 cm−3, which compares favorably with the theoretical value deduced from the band gap and the effective masses. From a Fermi‐level analysis, the dominant Cr acceptor is found to lie at 0.69±0.02 eV from the valence band. For many of the samples, the ionized‐impurity concentrations NI have been estimated from spark‐source mass‐spectrographic measurements and are compared with the concentrations predicted from μn. In general, the expected inverse relationship between μn and NI is found to hold, but the scatter in the data is quite large, mainly due to the uncertainties in the mass‐spectrographic results.