In an earlier paper, measurements of the electron mobility in GaAs as a function of electric field using a magnetoresistance technique were presented which revealed variations in the mobility due in part to field‐dependent scattering centers. These centers were hypothesized to be multiply ionized acceptor impurities which by their large Coulomb cross section adversely affected the electron mobility in the material. Further studies using photoconductivity techniques at liquid‐nitrogen temperatures to obtain the necessary sensitivity have shown that in the above case, acceptors, probably copper, exist in the material, and can be assumed to be the source of the observed field‐dependent effect. The relatively high purity of these materials, as evidenced by their good performance as microwave sources, indicates that the concentration of the impurity is of the order of 1012/cm3.