The temperature dependence of electron mobility of high‐purity LPE Ga1−xAlxAs layers (x<0.18) has been studied in the range 25–110 °K to measure the extents of alloylike and ionized impurity scattering. Values of the alloy scattering parameterEB(which may include a contribution from space‐charge scattering) determined for these samples were found to lie in the range 0.36–0.51 eV, the average being 0.44 eV. The compensation ratio was found to be about 2, and independent ofx.