Oxidized porous silicon is known to luminesce efficiently and with a ns‐fast response in the green‐blue region of the spectrum (2.3–2.7 eV) at room temperature. For microporous Si processed by rapid thermal oxidation at 1100–1200  °C in a dry oxygen ambient, we observe that the blue light has a time‐delayed component that indicates carrier trapping. In addition to the well‐established ultrafast light (∼1 ns) a signal with a time delay of order 1 s is present under UV photoexcitation. The time‐delayed blue light exists at low temperature and has an excitation onset edge at 4.3±0.1 eV, an energy that is usually associated with a band discontinuity at the Si–SiO2interface. ©1995 American Institute of Physics.