The electrochemical deposition of rhodium on Au(111) was studied by means of in-situ STM and cyclic voltammetry. A phase transition within the [RhCl6](3-) adlayer was found for Au(111) in 0.1 M H2SO4 + 10(-4) M RhCl3 + 10(-3) M HCl at 520 mV versus SCE, where a (root 13 x root 13)R13.9 degrees-superstructure of [RhCl6](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.