We have used scanning tunneling microscopy to investigate topographical changes on potentiostatically controlled semiconductor surfaces due to electrochemical reactions. The electrochemical processes reported here include Ni deposition on Ge(111), localized photoelectrodeposition of gold on GaAs(100) surfaces, and photocorrosion of GaAs. We have also carried out x‐ray photoelectron spectroscopy and secondary electron microscopy studies on samples that have been imaged with scanning tunneling microscopy.