In situ scanning tunneling microscopy (STM) was employed to examine the electrochemical etching process of an n-Si(111) electrode in dilute NH 4F solutions under potential control. Time-dependent STM images have revealed prominent effects of microscopic structures of Si on the rate of its dissolution. Multiple hydrogen-terminated Si atoms at the kink and step sites were eroded more rapidly than the monohydride Si step. This presumably resulted from the difference in reactivity of these hydrogen-terminated Si species. It is demonstrated that the density of kinks plays a main role in controlling the etching rate of Si. In the absence of kinks, not only the monohydride but also the dihydride steps were found to be stable. The etching rate of the monohydride step is substantially increased from a negligible value to 15 nm/min by the introduction of kink sites. The average etching rate for a dihydride step was 32 nm/min. Overall, the difference in the reactivity guides the dissolution of Si in a layer-by-layer fashion.