Brillouin scattering is used to determine the refractive indexN=n+i&kgr; of Ga0.52In0.48P layers, lattice matched to GaAs, in a wavelength range never investigated before (400 nm<&lgr;<800 nm). It is shown that the variations ofnare given by two different analytical expressions, depending on whether the corresponding photon energiesEare lower or higher than the band gapEg. Optical absorption measurements are deduced from the Brillouin linewidth. The whole set of elastic constant is also determined through the Brillouin shifts. ForE∼Eg, i.e., in resonance conditions,ndisplays a small but sharp peak closely related to the abrupt increase of the optical absorption. Besides the Brillouin investigations, complementary results obtained by ellipsometry and Raman spectroscopy confirm our optical properties measurements in Ga0.52In0.48P and the electronic resonance behavior, respectively. ©1995 American Institute of Physics.