Strongly coupled diffusion has been reported for fluxes of solutes that differ significantly in molecular size. According to the excluded-volume model, a solute increases the effective concentrations of other solutes by reducing the volume of solution they can occupy. The flux of solute 2 produced by the gradient delc(1), in the concentration of solute 1 is interpreted as the ordinary diffusion of solute 2 down its effective concentration gradient. Cross-diffusion coefficient D-21 = D(22)c(2)V(1eff)/(1-c(1)V(1,eff))(2) is predicted, where V-1,V-eff is the effective molar volume of solute 1. This model does not account for countercurrent coupled diffusion (D-21 < 0) and is found to be inconsistent with the Onsager reciprocal relation (ORR). A thermodynamic model of coupled transport is developed by approximating the flux of solute i as the product of its concentration, mobility, and chemical potential gradient driving force (-delmu(i)), which gives D-21 = D(22)c(2)(V-1 - V-0)/[1 - c(1)(V-1 - V-0)], where V-i is the partial molar volume of component i and the solvent is component 0. For dilute solutions with V-1 much greater than V-0, the thermodynamic prediction D-21 approximate to c(2)V(1)D(22) and the excluded-volume prediction D-21 approximate to c(2)V(1,eff)D(22) are qualitatively similar, but the thermodynamic model does not require the assumption of effective concentrations or effective volumes, provides a physical explanation for coupled diffusion, and is consistent with the ORR. Moreover, because partial derivativemu(2)/partial derivativec(1) is proportional to V-1 - V-0, the thermodynamic model suggests that a concentration gradient in solute 1 can drive co-current or counter-current flows of solute 2, depending on the relative volumes of solute 1 and solvent 0. These features are illustrated by comparing measured and predicted D-ik coefficients for solutions of n-octane(1) + n-hexadecane(2) in n-dodecane(0) at nine different compositions at 25 degreesC.