Transport properties of AlxGa1−xAs layers having AlAs mole fractions between 0.17 and 0.375 and grown by molecular beam epitaxy were investigated. The Al cell temperature was kept at 1075 °C while the Ga cell temperature was changed between 910 and 950 °C to obtain the aforementioned AlAs mole fractions. The donor levels of Sn in AlxGa1−xAs obtained from the temperature dependence of the net donor concentration are below 3, 30, and 40 meV forx=0.17,x=0.29, andx=0.375, respectively. Al0.22Ga0.78As layers having a net electron concentration of about 1.7×1018cm−3and grown at 630 °C exhibited a mobility of 868 and 1095 cm2/V sec at 300 and 78 °K, respectively. The mobility was found to be strongly dependent on the substrate temperature during the growth. The lower substrate temperatures resulted in lower electron mobility.