The doping of GaAs with small amounts of Bi has a profound effect on the properties of the alloy, most notably in the reduction of the bandgap. Yet the optical properties of GaAs1-xBix are not well known. Here we investigate the optical reflectance of GaAs1-xBix of He-Ne laser irradiation and employing a geminated ellipsoidal mirror apparatus to vary the incidence angle. We find that the refractive index determined in this way is lessened by the incorporation of Bi relative to the case of pure GaAs. We conclude this direct optical method may serve as a useful means of estimating Bi incorporation. © 2011 AOS.