A systematic study on the activation of photoluminescence from luminescent porous silicon nanoparticles (LPSiNPs) by oxidation in aqueous media containing sodium tetraborate (borax) is presented. The treatment promotes surface oxidation of the porous silicon skeleton and consequently generates an electronically passivated material. Photoluminescence is ascribed to quantum confinement effects and to defects localized at the Si-SiO2 interface, and the strong photoluminescence is attributed to passivation of nonradiative surface defects. The oxidation treatment (carried out at 20 degrees C) generates a gradual blue shift of the photoluminescence peak wavelength (from 800 nm to 630 nm), while the bandwidth remains relatively constant (approximate to 210 nm). During the treatment period, the external quantum yield ((ex) = 365 nm) of photoluminescence increases to a maximum value of 23% after 200 min, and then it decreases at longer treatment times. The decrease in photoluminescence intensity at longer times is attributed to degradation and dissolution of the nanoparticles, which is inhibited at higher nanoparticle concentrations or by addition of free silicic acid.