Methyl viologen (MV(2+)) has been studied at a thin mercury film electrode in the presence of iodide ions. Conventional electrochemistry and in-situ optical probes have been applied to investigate processes not previously observed in fluoride solutions. As the potential is made negative, iodide desorption is seen attended by an intense increase in reflectivity. The MV(2+) first reduction coincides with the formation of the (MV(.+)-I-) species on the surface, at the monolayer level, showing strong adsorbate-adsorbate interactions and a flat orientation of the pyridyl groups. The ac current response accompanying the appearance of the (MV(.+)-I-) film was found to be similar to that of a phase transition (condensation) in the adsorbed state. The dissociation of(MV(.+)-I-) clusters, to yield MV((solid))(0) + I-(aq)(-), at the onset of the second reduction wave of MV(2+), leads to a reflectivity enhancement in the in-phase optical signal paralleled by a sharp doublet in the imaginary current response. The splitting of the reduction peak of (MV(.+)-I-) may arise from different orientations of similar clusters or from distinct oligomer sizes.