We conducted a series of experiments on porous silicon (PS) and porous polysilicon (PPS) to investigate the origins of photoluminescence in these materials. A study of the effects of surface layer plasma etching, for several different durations, on the as-anodized samples was conducted. For the photoluminescence (PL) study, results show that the intensity decreases rapidly, but the peak locations remain fairly unchanged, as the surface etching proceeds. In addition, only one band centered at around 680 nm (1.82 eV) is found in the PS samples whereas two bands, centered at 400 nm (3.1 eV) and 680 nm, are found in the PPS samples. The 400 nm peak disappears when the surface oxide layer was completely removed. These results could not be explained with the quantum confinement model of silicon nano structures. We ascribe these PL effects to the radiative centers in the surface oxide layer. Fourier transform infrared (FTIR) measurements on these samples further reveal that the PL peak at 680 nm correlates well with the NBOHCs (non-bridged oxide hole centers or /spl equiv/-SiO/spl middot/) and the 3.1 eV peak can also be attributed to the oxygen vacancy (/spl equiv/Si-Si/spl equiv/) in the surface silicon oxide layer.