Topochemical etching processes of indented silicon {111} surfaces were observed in detail by using aqueous solutions of hydrofluoric acid with controlled pH. Specific preference of a local dissolution reaction with respect to dislocations, concentrated near the slip bands, was diminished with increasing pH. Dissolution was concluded to be a two-step process, i.e., the surface oxidation at high dislocation density due mainly to the dissociative adsorption of dissolved oxygen, followed by the dissolution of oxides attacked mainly by HF2- ions. These mechanisms were explained by virtue of a band model with a higher localized band of electrons which belong to the dangling bonds along the dislocation, as compared with the band of adsorbed oxygen.