In this work we present a method for monitoring the optical parameters of a film during its etching process. Optical interferometry of two laser beams with different angles of incidence is observed to measure the index of refraction profile and the etch rate evolution simultaneously. With this technique we have measured the inhomogeneity in the etch process of porous silicon layers, which is an essential issue to make good quality optical microcavities or photonic crystals with this material. In addition, by sweeping a range of currents we are able to fully characterize the etch rate and the porosity vs. current density in one single sample, without the need of independent measurements. (C) 2003 The Electrochemical Society.