A Nernst diffusion-layer model is developed for the study of reactant diffusion-migration in electrolyte solutions with coupled chemical-electrochemical reactions. Specifically, proton reduction to hydrogen gas and the Kolbe oxidative dimerization of acetate to ethane and carbon dioxide are studied in an acetic acid solution with supporting electrolyte : an inert (LiClO4) and a reactive electrolyte (NaOH) are each considered. The model predicts concentration and potential profiles within the diffusion layer and the mass-transfer limited current density as a function of the ratio of the supporting-electrolyte concentration to that of acetic acid. For proton reduction, the ratio of the limiting to diffusion current increases with decreasing acid strength in the limit of zero supporting-electrolyte, while the opposite is true for the Kolbe oxidative dimerization of acetate. For the Kolbe oxidative dimerization of acetate in sodium hydroxide, a maximum exists in the ratio of the limiting to diffusion current when the concentration of sodium hydroxide is approximately equal to that of acetic acid. The maximum is a result of the weak-acid strong-base chemistry. Numerical calculations for proton reduction from acetic acid as a function of the supporting-electrolyte concentration are compared with published experimental data. The numerical results and experimental data are in agreement when there is an excess of supporting-electrolyte present, but diverge as the supporting-electrolyte concentration approaches zero.