A new technique is applied to determine the sites of implanted substitutional impurities in GaAs. Since theZvalues are nearly equal, Ga and As substitutions cannot be distinguished from a measurement of the width of channeling dips, but the asymmetry of a <110> scan in the {110} plane is opposite for backscattering from the two types of atoms, and a comparison with the observed asymmetry for backscattering from the impurity determines the site. As expected, In is found to replace Ga and Sb to replace As. In addition, the preferential site of implanted Sn atoms is shown to be a Ga site. These results confirm earlier assignments based on isomer shifts in Mössbauer experiments.