The atomic structure of H-terminated Si(111) surfaces is investigated by in-situ STM and electrochemical measurements in NH4F solutions of 2 < pH < 8. Etch rate measurements show only a slight pH-dependence between pH 2 and 14, when including alkaline solutions. Electrochemical results indicate that the etching comprises two components, one chemical and the other electrochemical, whose relative importance depends on the pH. The possible reactants involved in the etching are studied by varying the composition of the solutions. Models describing the surface processes at the molecular level are presented. The main conclusion of this work is that the nature of the chemical etching tends to smoothen the surface, as is the case in buffered ammonium fluoride where ideally flat surface can be prepared, whereas the electrochemical one roughens the surface.