Photoluminescence (PL) of porous silicon after rapid thermal oxidation was studied using excitation energies of 5.0 and 6.4 eV. The emission spectra in both cases are dominated by a broad blue band centered at 430-450 nm and a much weaker red band positioned at 680-720 nm. Using atomic force microscopy (AFM), we have found a correlation between blue PL intensity and increase in feature size caused by progressive oxidation. The blue luminescence, found in porous Si only after oxidation, is identical, with respect to spectrum and fast decay, to that of thermally grown oxide on crystalline Si. Based on these findings, we conclude that the blue luminescence originates from SiO2 rather than from Si nanocrystals embedded in the oxide matrix. The red luminescence, on the contrary, has a different origin. The intensity of red band is a function of oxidation temperature and at RTP above 1050 °C, the red band is completely eliminated. The red luminescence characteristics are discussed in relation to the possible mechanism.