Most samples of GaAs show properties similar to those of germanium and silicon, but it is possible to prepare GaAs with a resistivity at room temperature greater than 106ohm‐cm, and the electrical properties are then more like those of the wide band gap II–VI compounds, such as CdS. This form of material, known as semi‐insulating GaAs, previously has not been studied thoroughly, partly because homogeneous samples were not available.Measurements have now been made on semi‐insulating GaAs, and results are reported for carrier concentration and mobility as a function of temperature. The interpretation of the results is sometimes complicated because even at room temperature the activation energy is about half of the intrinsic activation energy, and the carrier concentration can be close to the intrinsic concentration.The dominant lattice scattering mechanism in GaAs is believed to be polar scattering, but even in the purest samples of semiconducting GaAs made thus far, impurity scattering is observed at room temperature. In a highly compensated material like semi‐insulating GaAs, neither the Brooks‐Herring nor the Conwell‐Weisskopf theory of impurity scattering is likely to be valid. An initial study of carrier scattering has been made using measurements of transverse magnetoresistance and the field dependence of Hall coefficient. Some values for carrier lifetime are also reported.