We present photoluminescence (PL) and infrared absorption on sets of porous silicon with peak energy ranging from 1.5 to 1.9 eV prepared at different anodization current density. The effects of the as-grown samples under several different treatments have been investigated. Quite surprisingly, the PL peak positions of all the samples studied shift toward 1.7 eV upon the introduction of more oxygen by annealing in a low-temperatureH2environment then exposing to the air. This behavior indicates that the 1.7 eV emission plays a unique role in the PL spectra of porous silicon. However, samples rinsed in water show only blueshift. From our measurements, we conclude that PL emission in porous silicon contains two components. One is the above-mentioned 1.7 eV peak, which can be correlated to the absorption of O–Si–H bonds. The other component of PL is shown to be sensitive to the strength of the Si–O–Si bond related absorption. Based on the previous reports and the results shown here, a possible PL mechanism in porous silicon is emerging. We propose that PL originates from quantum confinement effect for samples with peak energy lower than 1.7 eV. For samples with PL peak larger than 1.7 eV, the PL results from the fact that after generated in the nanocrystalline silicon by photon pumping, the carriers relax into the surface states and then make a radiative recombination. We studied the evolution of the PL spectra under laser illumination and found that the anomalous behavior in the evolution process can also be explained with our model. ©1997 American Institute of Physics.