Reflectance difference spectroscopy (RDS) is applied to the study of optical anisotropy in spontaneously ordered GaInP grown by metalorganic chemical-vapor deposition. The degree of ordering in GaInP has been associated previously with a shift of the band-gap energy DELTAE0, and a crystal-field valence-band splitting A(C). Theoretically and experimentally, both quantities are approximately proportional to the square of the order parameter, which varies from 0 to 1 for disordered and perfectly ordered GaInP, respectively. In this study, we examined a number of GaInP layers grown under conditions that yield a wide range of band-gap energies. The main spectral feature in all samples is a bulk-related, asymmetric peak at E0 with a long tail that extends well below E0 and a sharp, high-energy cutoff at E0 + DELTA(C). The intensity of this peak is proportional to square-root DELTAE0 and is therefore linear with the order parameter. By annealing GaInP in PH3/H-2 mixtures, we find that the RD spectral features for energies between E0 + DELTA(C) and 3 eV are mainly surface induced. Evidence for a bulk-related RDS peak at E1 is also found.