The location of substitutional impurities in GaP (Sb and In) has been investigated by channeling. The sites may be determined from the width of ⟨110⟩ backscattering dips, but a new technique based on asymmetry of the channeling dips is developed, which appears to be more powerful for compounds with ZnS structure. It requires smaller ion doses and may be applied also to compounds such as GaAs with nearly equal atomic number of the compound constituents. The technique may also be applied to determine the stacking order in a ⟨111⟩ direction of such compounds.