Co and Co‐P alloy films were electrodeposited on a rotating disk electrode. The experimental results suggested that the ratio of partial current density consumed by phosphorus deposition to the total current density was relatively small and that the competition of electrodeposition of Co2+ ion and gas evolution determined the phosphorus content in the Co‐P film. The addition of phosphorus acid to electrolyte solution was also found to depress the preferential growth of the (100) plane, which was observed in the electrodeposition of Co film, and induced the growth of (002) plane. A further addition of phosphorus acid resulted in the formation of amorphous‐like film. The diffusion model, taking into account the dissociation of molecules, was applied to calculate the pH value at the cathode surface. The surface concentrations of Co2+and were also estimated from the partial current densities. The phosphorus content of the electrodeposited film has been correlated to the surface concentration of based on a concept of adsorption equilibrium of . Adsorbed molecules may hinder the surface diffusion of Co atoms, which causes the formation of the amorphous‐like structure of the electrodeposited film.