The role of finite-rate homogeneous chemistry during the electrodeposition of binary alloys is analyzed using a detailed mathematical model. The model accounts for solute reactions and transport within the aqueous boundary layer as well as multistep reactions on the electrode surface. Calculations are carried out for nickel-iron deposition from a sulfate bath, using a previously proposed surface-reaction mechanism that accounts for the observed anomalous codeposition. The common assumption of homogeneous chemical equilibrium is found to produce only modest errors in the computed deposition rates, but it distorts the relative importance of hydrolyzed metal ions. In contrast with previous literature claims, the present calculations show that these ions tend to have a greater role than is predicted by assuming equilibrium. The model also suggests that the precipitation of solid metal hydroxides may be slow, i.e., that substantial supersaturation of the plating solution may occur. (c) 2007 The Electrochemical Society.