Thin nanoporous Si layers were formed onn‐Si(100) in 0.1 M NH4F electrolyte and investigated with field emission scanning electron microscopy and photoluminescence (PL). The microstructure of the electrochemically prepared Si surfaces depended strongly on thepH of the electrolyte. A pebblelike surface structure was formed during anodization atpH 4.5, while nanoporous Si layers were formed at lowerpH. The onset of macropore formation was observed after anodization atpH 3.0. The shape of the PL spectra was found to be practically independent of the nanoporous structure. The results are discussed on the basis of the etch rate of oxidized Si atoms and of the role of surface states for PL. ©1995 American Institute of Physics.