Abstract The complex refractive index profile of a damaged layer was obtained using ellipsometry combined with chemical etching. Samples of GaP were implanted with arsenic ions initially at an energy of 180 keV to a dose of 3 × 10 15 cm -2 and then at an energy of 50 keV to a dose of 1 × 10 15 cm -2. From the optical point of view an implanted single crystal of GaP presents an inhomogeneous absorbing film on a transparent substrate. This system is, in general, covered with a very thin passive film whose composition depends on the preparation of the surface. The standard evaluation of optical constants from a series of ellipsometric parameters measured during successive stripping is complicated owing to the poor reproducibility of this passive film. In the method used the final optical constants were evaluated using several successive approximations. The profile obtained was compared with a theoretical damage distribution.