A mathematical model, predicting the influence of a citrate complexant in the electrodeposition of copper-nickel alloys, was developed. The model which takes into account the deposition of complexed and uncomplexed species is based on convective-diffusion transport and Butler-Volmer reaction kinetics. Cu-Ni alloys were electrodeposited onto a rotating cylinder electrode at different rotation rates from electrolytes containing different total citrate concentrations. The theoretical model was found to predict correctly the influence of experimental variables, such as citrate concentration and rotation rate on the polarization behavior, alloy composition and current efficiency. Increasing the total citrate concentration in the electrolyte resulted in a reduction of the electrochemical reaction rates, which is explained by the interactions between the complexed and uncomplexed reacting species.