Secondary ion mass spectroscopy was used to study the incorporation of deuterium into Si during the formation of porous silicon in fluoride D2O-based solutions. Results show that the deuterium diffuses from the pore tips towards the bulk silicon and remains in the pore walls in a high concentration. Measurements of the thickness of porous silicon suggest that the penetration of H species is a necessary condition for the porous Si layer to form. A simple model illustrates how the initiation of pores occurs by selective dissolution of H-induced structural defects.