A method using coincident rf sputtering and rf discharge decomposition is shown capable of producing single‐crystal, epitaxial, nonequilibrium solid solutions of Si in GaAs with the composition of Si far exceeding the limits reported for the bulk equilibrium phase diagram. Measured values of alloy composition and lattice parameter indicate close correspondence to Vegard's law. High‐temperature annealing of epitaxial films demonstrates the ultimate instability of the alloys.