The kinetics of the reaction of silicon with solutions of hydrofluoric acid, nitric acid, and acetic acid are reported as a function of the composition of the etchant. The system qualitatively behaves in the same manner as the system hydrofluoric acid, nitric acid, and water, which has been reported previously. Quantitatively, the acetic acid diluted system shows a much higher tolerance for the diluent than does the water diluted system. The greater tolerance for acetic acid has been explained on the basis of the lesser ionization of nitric acid in acetic acid than in water. The reaction mechanisms postulated for the water diluted system have been found to apply equally well to the acetic acid diluted system.