(GaAs)(1-x)(Si-2)(x) metastable alloys were epitaxially grown on (001), (111), (110), and (112) GaAs. Single-crystal alloys were obtained for Si concentrations in the range 0less than or equal toxless than or equal to0.43. At higher concentrations the Si segregated. The long-range order parameter, for each growth direction studied, was determined as a function of Si concentration by high-resolution x-ray diffraction. The behavior of this parameter with Si concentration is influenced by growth direction. This fact provides direct evidence that the substrate geometry affects the atomic ordering of these alloys. The results obtained from these alloys provide additional support to the validity of the proposal that the growth direction influences the order-disorder transition observed in other alloys of this kind.