In situ scanning tunneling microscopy (STM) and atomic force microscopy (AFM) were employed to examine the surface morphology of a Si(lll) cathode in 1% HF or a mixed HF and H2O2 solution under potential control. In the absence of H2O2, in situ STM revealed island-like features due to the native defects at the Si substrate and the subsequent dissolution of the native SiO2 layer by the HF etchant. Atomic resolution was possible, and a well-ordered hexagonal array, presumably the Si(lll)-(1 × 1):H phase, was imaged by STM after the oxide layer was removed. This long-range ordered structure was stable in dilute HF for ca. 1 h before it became disordered by the evolution of H2. In a mixed solution of H2O2 and HF, depending on the relative concentration of these two species, either the oxide formation or its subsequent dissolution dominated. When the former reaction was more important, the STM tip physically scratched the insulating SiO2 layer. At higher relative HF concentrations, a long-range ordered one-dimensional fiber structure arranged in a sawtooth pattern was imaged, when the dissolution of SiO2 occurred. These surface features are attributed to chemical etching of Si, possibly the metastable SiOH surface species.