The electrochemical deposition of rhodium on Au(111) was studied by means of in-situ STM and cyclic voltammetry. A phase transition within the [RhCl 6 ] 3− adlayer was found for Au(111) in 0.1 M H 2 SO 4 +10 −4 M RhCl 3 +10 −3 M HCl at 520 mV versus SCE, where a ( 13× 13)R13.9°-superstructure of [RhCl 6 ] 3− is formed on the gold substrate. Rhodium starts to deposit irreversibly around 200 mV versus SCE. First a rhodium bilayer grows, the electrochemical behaviour of which is similar to that of a well-ordered Rh(111) surface. On top of the second rhodium monolayer the formation of small, three-dimensional clusters is observed. The electrocatalytic properties of rhodium films with an average thickness up to ten monolayers were studied by using CO oxidation as a test reaction. The influence of the surface structure on the CO oxidation kinetics is discussed briefly.