Porous Si (PS) layers are prepared by stain etching in a HF/NaNO2solution on bothp‐ andn‐type crystal Si substrates, and are characterized by photoluminescence (PL), electron paramagnetic resonance (EPR), and infrared absorption (IR) spectroscopy. The PL spectra under 488 nm laser excitation exhibit a strong peak at 680–720 nm for various samples of different substrate parameters and remain stable upon aging in air or &ggr; irradiation; as‐etched (∼20 min in air before measurement) and aged (for up to six months) samples show no detectable EPR signal but the &ggr;‐irradiated samples show an isotropicg=2.006 signal of peak‐to‐peak linewidth of 1.1 mT supporting an amorphous Si structure; the IR spectra show both hydrogen and oxygen related IR modes in the as‐etched samples and the former decreases with aging time in air while the latter increases. Comparing our results with those of anodically etched PS samples we conclude that: (1) the PL peak position of the stain PS seems to be unique and stable as compared with that of the anodic PS varying in 620–830 nm; (2) the isotropic EPR signal of the stain PS reflects no crystallinity, in contrast with the anisotropic signal of the anodic PS; and (3) obvious oxidation in the as‐etched stain PS is also in contrast with the nonobservation of oxygen‐related IR modes in the as‐etched anodic PS. We discuss the results in terms of structural properties and PL mechanism of PS.