In this paper we investigate the oxidation of porous silicon by O-3, H2O2, and, for comparison, in normal air. Such an oxidation may serve as passivation for porous silicon in applications in order to prevent devices from degradation. The changes in the dielectric function caused by this oxidation was monitored by spectroscopic ellipsometry. Application of both H2O2 and O-3 resulted in a significant lowering of the values of the imaginary part of the dielectric function as expected when oxidizing the inner surfaces of these layers. For a multilayer structure we show that ozone treatment of this structure indeed passivates that sample against further oxidation in air as studied over an extended period of time (3 months).