In situ scanning tunneling microscopy (STM) has been used to investigate the bias dependence of the etching of Si(111) in alkaline solutions. Sequences of images showing the time evolution of the surface structure have been recorded. Under hydrogen evolution, double layer steps (∼ 3 Å) are preferentially etched laterally. Closer to the rest potential, corrosion pits ( ∼ 3 Å deep, i.e., one Si double layer deep) are nucleated on wider terraces, increasing the etch rate. Individual atoms could also be resolved on terraces, showing that the surface of Si(111) is unreconstructed when in contact with an aqueous solution. The results presented here confirm previous electrochemical data concerning the bias dependence of the etch rate and yield new insight into the etch mechanism of Si in such solutions.