The microtopography of n-Si (111) in acidic fluoride-containing solution is strongly influenced by the electrode potential under illumination. The surface condition changes between flat, rough and porous, respectively. The thickness of the hydrogen-terminated porous silicon layer depends on the photocurrent density and reaches a saturation value, where the porosity is preserved. The surface roughness and porosity during the photoelectrochemical etching process was investigated by in-situ Fourier transform infrared spectroscopy using multiple internal reflection techniques due to the low IR absorption of the Si-H stretching mode. Additional single internal reflection measurements show no formation of silicon oxide during the light-assisted corrosion reaction, The surface topography was inspected ex situ by field emission scanning electron microscopy with a resolution of about 2 nm.