Anisotropy in the above-band-gap optical reflectance along the [110] and [110] directions has been used to investigate long-range atomic ordering in metastable epitaxial (GaAs) & (Si~) (001) alloys as a function of Si concentration x. The amplitude of the differentiated reAectance-difference signal was found to decrease monotonically with increasing x and reach zero, corresponding to the critical concentration for the zinc-blendetodiamond transition, at x =0.37, consistent with x-raydiffraction results. The highest sensitivity in the optical-anisotropy spectra was obtained in the spectral region near the Eo critical point (4.5 eV in GaAs).