A novel plating solution for CoPt alloys electrodeposition containing CoSO4 as a source of Co and H2PtCl6 as a source of Pt, has been developed. The solution was stable for more than one year. It was proposed that the most abundant six-coordinate (PtL62-)-L-IV electroactive complex in new plating solution is PtCl5(OH)(2-). The electrochemical conversion of (PtL62-)-L-IV -> (PtL42-)-L-II, occurring through two electron transfer and two ligand loss, was studied by using several electroanalytical techniques. It was concluded that the mechanism of the electron-transfer-PtL-bond-breaking process in the electrochemical reduction of (PtL62)-L-IV- complex is a function of electrode materials (Pt, Ru). On a Pt electrode the reduction through stepwise pathway occurs, while on Ru the concerted mechanism prevails. The composition of CoPt alloys is strongly dependent on potential and agitation of the solution. Electrodeposition of CoPt alloys is demonstrated at a potential positive to that required to deposit elemental Co, which is attributed to the negative enthalpy of CoPt formation. (C) 2015 The Electrochemical Society. All rights reserved.