Porous silicon exhibits a fast (similar to 1 ns) luminescence band in the blue-green (2.3-2.7 eV) spectral region when post-treated by rapid thermal oxidation. In addition we observe for the blue emission a time-delayed signal with a typical time constant of order 1 s that indicates carrier trapping. This component is present at low temperatures and has a sharp excitation onset at 4.3 eV. Quenching of the slow signal after additional thermal treatment of aged samples is accompanied by changes in the IR spectrum.