Vacuum annealed and oxidized porous silicon layers (PSL) were investigated by in situ spectroscopic ellipsometry (SE). The nominal porosity of the layers was between 60 and 77% and the nominal thickness was 500 nm. The annealing was performed by direct ohmic heating (R.T. < 450 degrees C) in 5 x 10(-10) Torr vacuum. The oxidation was performed in two steps, the first step at 5 x 10(-5) Torr, the second at 10 Torr. Two optically different types of silicon compounds, a bulk-type silicon (c-Si) and a fine-grain polycrystalline silicon with enhanced absorption due to extensive grain-boundary regions (p-Si) were mixed with voids in the appropriate ratio to fit the spectra of as-prepared PSL. For the annealed PSL amorphous silicon (a-Si) was needed in conjunction with p-Si. The oxidized PSL could be fitted with a reduced a-Si content. We can interpret the annealing effect as a depassivation process of the inner surfaces of the PSL (a-Si fraction). At the same time, oxidation leads to a repassivation process.