Zunger et al. suggested that the creation of a subsurface selectivity for occupation by a small cation under the strained anion dimer rows, and occupation by a large cation underneath the opening between dimer rows is the main thermodynamic driving force for CuPt ordering in GaInP alloys. The present calculations support this picture for AlInP. All stable P-dimer structures identified here are characterized by P dimers that form above Al, while In occupies the corresponding subsurface positions between the P-dimer rows, see Fig. . No such correlation is found for the hetero-dimer structures. Mixed anion-cation dimers form on top of In, e.g., in the 2×2-2InMD-2H structure, as well as above Al, e.g., in the 2×1-AlMD-2H structure. This agrees with the observation that the degree of atomic ordering of MOVPE-grown AlInP depends sensitively on the preparation conditions .