Nernst-Planck equations are used to calculate ternary interdiffusion coefficients (D-ik) for aqueous solutions of a molecular weak acid and base that react as follows : B + HA reversible arrow BH+ + A(-). The D-ik coefficients provide a convenient description of diffusion by relating the fluxes of the total acid and base components to the gradient in the concentration of each component : J(1) = -D11VC1 - D12VC2; J(2) = -D21VC1 - D22VC2. Cross-coefficients D-12 and D-21 measure the coupled fluxes of acid and base caused by local equilibrium of the proton exchange reaction and by the diffusion-induced electric field that adjusts the speeds of the diffusing BH+ and A(-) ions to prevent charge separation. The contribution to each D-ik coefficient from the migration of the BH+ and A(-) ions in the electric field is evaluated to help interpret the concentration dependence of the diffusion behavior. Strongly coupled diffusion is predicted in cases where a gradient in the acid is formed in an alkaline solution of excess base, or vice versa. The accuracy of the predictions is tested by using Taylor dispersion to measure D-ik coefficients for aqueous solutions of acetic acid + triethylamine at 25 degrees C.