The relation between mutual diffusion in binary mixtures and collective diffusion in colloidal suspensions is studied on the basis of irreversible thermodynamics. Emphasis is placed on the relation between the gradient of the chemical potential driving mutual diffusion and the osmotic pressure gradient and its dependence on relative particle size. The analysis is illustrated by a study of ideal solvent mixtures, isotopic mixtures, and hard-sphere mixtures. It is shown that the two Onsager transport coefficients characterizing mutual diffusion of both components are related by a symmetry relation. (C) 2003 American Institute of Physics.