As-prepared low resistivity p-type porous silicon (PS) and PS aged for 15 months are subjected to etching treatment in 0.05% aqueous NaOH at room temperature. Photoluminescence (PL) properties of the treated PS are correlated with chemical states and microstructures analyzed by Fourier transform infrared spectroscopy and scanning electron microscopy. The red PL from as-prepared PS is almost quenched by NaOH treatment for 10 s, while a lightly oxidized outer surface layer of about 100 nm is removed. The red PL from aged PS becomes weak gradually as the Si-O bond content in PS decreases with NaOH treatment time. The aged PS shows strong PL even after the treatment for 450 s, while the heavily oxidized outer surface layer partially remains. These results suggest that the oxidized outer surface layer in PS is responsible for the red FL.