In this paper we describe the "composition gradient" effect in polycrystalline CoPt films, which results from an initial increase of Co content during the controlled potential electrochemical deposition at short pulses (10-100 s) from quiescent solutions. At longer times of electrodeposition (100-300 s) the CoPt composition reaches steady-state values. The electrodeposition of Co/Pt represents a case of magnetic alloy deposition of the type FM/NM with a ferromagnetic metal (FM = Fe, Co, Ni) and a nonmagnetic metal (NM = Pt, Au, Ag, Pd, Cu) with widely separated reduction potentials. The measurements of partial current densities provided a key insight to understanding the origin of the "composition gradient". The results show: (i) practically independent cobalt (iCo) partial current density throughout deposition, and (ii) decrease of platinum (iPt) and hydrogen (iH) partial current densities. The theoretical analysis of CoPt electrodeposition at the controlled potential was used to compare experimental and calculated data. The electrochemical behavior of hexachloroplatinate solutions, as well as the crystal structure and morphology of CoPt films have been studied. (C) The Author(s) 2016. Published by ECS. All rights reserved.