A model of a zinc-blende—diamond order-disorder transition is proposed and applied to (GaAs) 1− x Ge 2 x substitutional crystalline alloys. In disordered Ge-rich alloys, the stable phase is one in which either Ga or As atoms can occupy nominal cation and nominal anion sites with approximately equal probabilities; in ordered GaAs-rich material, Ga (As) atoms preferentially occupy nominal cation (anion) sites. A three-component spin Hamiltonian, mean-field theory, and empirical tight-binding theory are all used in conjunction to predict equilibrium phase diagrams and the dependence on alloy composition x of the direct band-gap energy E 0 (x). The theory accounts for the observed V-shaped dependence of E 0 (x) in (GaAs) 1− x Ge 2 x and for several qualitative facts concerning the growth of these interesting metastable crystalline alloys.