The dependence of electron mobility &mgr; on temperatureTin GaAs and AlxGa1−xAs indicates that for compensated material a term having &mgr;∝T−1/2causes a significant reduction in the mobility measured at high temperatures. The magnitude of theT−1/2term in mobility, denoted &mgr;CA, is found to be linearly proportional to the compensating acceptor concentration over a range of more than two orders of magnitude in samples with no intentional doping where carbon is the major compensating acceptor. Intentional compensation using Ge and Zn is found to have no effect on &mgr;CA. Illumination (h&ngr;≳EG) has no effect on &mgr;CA. Such illumination is demonstrated to significantly reduce the size of space‐charge layers at then‐iinterface. Thus, theT−1/2mobility isnotdue to scattering by space‐charge regions as has been previously assumed. The acceptor C, or an associate involving C, is concluded to be the scattering center responsible for &mgr;CA. The effect may be due to the short‐range central‐cell potential resulting from the large electronegativity difference between C and the As for which it substitutes in the lattice.