Recent “in situ” observations of electrochemical oxidation of noble metals and group-VIII metals by means of scanning tunneling microscopy (STM) have been reviewed. Single crystal surfaces of gold and platinum are oxidized by anodic polarization in acidic media, resulting formation of amorphous oxides, which can be reduced by cathodic polarization. The metal atoms involved in oxidation processes are transported along the surface and thus the surface morphology changes. Single crystal surfaces of nickel and cobalt, after proper cleaning procedure, exhibit formation of wellordered epitaxial oxide layers in slightly acidic media. The lateral lattice structures of the oxide layers are close to lowindex planes embedded in the corresponding oxide crystals. Formation of stoichiometric oxides as well as hydrooxides is evidenced. Passivation takes place on Ni surfaces due to the oxides, whereas continuous anodic dissolution occurs on Co surfaces covered by oxide layers.