The preparation and surface treatment of a rhodium single crystal with (111) orientation are described. Surface characterization by scanning tunnelling microscopy (STM) in air, including topography and atomic-scale pictures, is reported. Hexagonal surface domains one atomic diameter thick are observed with edges parallel to the dense rows of the basal orientation; three or four atomic levels are included in this distribution. The atomic resolution reveals a surface lattice with the 2 x 2 periodicity, probably formed by oxygen adatoms. The voltammetric electrochemical study was carried out using perchloric acid solution which is known to give a reduction of the perchlorate anion at an Rh electrode. The reduction of perchlorate is revealed at low sweep rates. The effect of the formation of chloride anions or other anionic reduction products on the voltammetric response of this electrode was investigated in detail. A dramatic effect was found regarding the unusual reversible adsorption state around 0.6 V, which is very often compared with the unusual adsorption state of Pt(111) in the same solution. A quantitative analysis of the various adsorption states is given. The unusual state is shown to be sensitive to the electrochemical oxidation of the surface above a potential threshold of 1.080 V (RHE), similar to the behaviour of Pt(111). The presence of this state is thus shown to be characteristic of two-dimensional long-range order.