The determination of the mutual-diffusion coefficients of a quaternary Xerox ink system is an extremely complicated process. Experimental measurement of the nine mutual-diffusion coefficients is extremely difficult, and data for only a few quaternary systems are available in the literature. On the other hand, the determination of the self-diffusion coefficients is relatively easy if NMR is applicable. For the conversion of self to mutual-diffusion coefficients, three different approximations are proposed. Two of the cases involve the need for a thermodynamic equilibrium relationship, which was calculated from the UNIFAC model. With the mutual-diffusion coefficients, a drying model can be developed for the ink system.