The dimer [Ru(bpy)(CO) 2 Cl] 2 dissociates in DMSO electrolyte to form free chloride and DMSO-coordinated monocation with an equilibrium constant of ca. 6×10 −4 moldm −3 . Neutral dimer is oxidised with peak potential E p =+0.75V (vs. Ag/AgCl, c KCl =3.4moldm −3 ) in a one-electron process. The monocation (reduced in one-electron process with E p =−0.96V) is able to reversibly dissociate into dicationic dimer, reduced with E p =−0.85V. Mostly monocationic tetramer is formed during electrolysis at −1V which in turn is reduced to form monocationic octamer with E p =−1.09V. EQCM, RRDE and voltammetric data indicate that reduction of the octamer results in hexadecamer formation and deposition in a process with E p =−1.14V. A model for the reduction voltammetry was tested using digital simulation. UV–vis and voltammetric results show that tetramers are formed in the oxidative break-up of the Ru–Ru bonded film. These tetramers can be oxidised back to dimers in two one-electron processes via a stable intermediate. Such dimers can be back-reduced to form film.