The bandgap, surface Fermi level, and surface state density of a series of GaAs 1-xSb x surface intrinsic-n + structures with GaAs as substrate are determined for various Sb mole fractions x by the photoreflectance modulation spectroscopy. The dependence of the bandgap on the mole composition x is in good agreement with previous measurements as well as predictions calculated using the dielectric model of Van Vechten and Bergstresser in Phys. Rev. B 1, 3551 (1970). For a particular composition x, the surface Fermi level is always strongly pinned within the bandgap of GaAs 1-xSb x and we find its variation with composition x is well described by a function E F 0.70 - 0.192 x for 0 x 0.35, a result which is notably different from that reported by Chouaib [Appl. Phys. Lett. 93, 041913 (2008)]. Our results suggest that the surface Fermi level is pinned at the midgap of GaAs and near the valence band of the GaSb.