We show that single-phase face-centered cubic (fcc) CoxPt1-x can be electrodeposited from solution containing cobalt(II)sulfate and diamminedinitritoplatinum(II). The composition and crystal structure of the films are dependent on solution composition and deposition potential. For films deposited at constant current, the film composition is only weakly dependent on the Co(II)/Pt(II) ratio. However, the film composition is strongly dependent on the deposition potential. At intermediate potentials, films with a mole fraction of cobalt up to x = 0.52 are single-phase fcc CoxPt1-x solid solutions. At more negative deposition potentials phase separation leads to the formation of a two-phase alloy with disordered hexagonal close-packed CoxPt1-x and fcc CoxPt1-x solid solutions. (C) 2004 The Electrochemical Society.