We present evidence, from theory and experiment, that ZnSnN2 and MgSnN2 can be used to match the band gap of InGaN without alloying—by exploiting cation disorder in a controlled fashion. We base this on the determination of S, the long-range order parameter of the cation sublattice, for a series of epitaxial thin films of ZnSnN2 and MgSnN2 using three different techniques: x-ray diffraction, Raman spectroscopy, and in situ electron diffraction. We observe a linear relationship between S2 and the optical band gap of both ZnSnN2 (1.12–1.98 eV) and MgSnN2 (1.87–3.43 eV). The results clearly demonstrate the correlation betweencontrolledheterovalentcationorderingandtheopticalbandgap,whichappliestoabroadgroupof emerging ternary heterovalent compounds and has implications for similar trends in other material properties besides the band gap.