The chemistry of silicon surfaces in aqueous fluoride solutions is complex and exhibits many unique features. The etch rate of the hydrogen-passivated surface passes through a maximum at about pH 7 and is slow at low and high pH. The density of etching intermediates exhibits a sigmoidal behavior of low density in acidic and high density in basic solutions. Pie show that the pH dependence of both the etch rate and the density of intermediates can be explained by taking into account the fluoride ion concentration, and the concentrations of molecular HF, the dimer (HF)(2), HF2- as well as OH- and H2O.