In situ real-time measurements of etching processes by infrared total reflection spectroscopy were carried out for the first time on Si(lll) and Si(100) surfaces in ammonium fluoride solution. The absorption bands became broad by the interaction between terminal hydrides with water molecules. On Si(lll), the band at 2083 cm(-1) continuously grew and was sharpened by the formation of monohydride on the ideal Si(lll) with the decrease of defects. On Si(100), dihydride first appeared, which was subsequently replaced by monohydride on steps produced as a result of etching. The stepped surface was continuously etched, and dihydride on Si(100) again increased with pyramid-like protrusions having (111) facets with many steps. The pyramids kept growing at the expense of dihydride to be the dominant structure on the surface.