The diffusion of molecular species within a sulfuric acid–water system has been monitored by Raman spectroscopy, a thermodynamic-chemical model of the mass transport properties of the species has been established, and its parameters optimized. It has been shown that the non-ideality of this multicomponent system plays a crucial role in its mass transport properties, which have been explained in terms of a diffusion model for the molecular species. The individual effective diffusion coefficients are not constant (characteristic of ideal systems) but are a function of the concentration of the species in solution. The model has been conceived in such a way that it can be adapted to any multicomponent mixture when the equilibriums among the ions are known. Raman spectroscopy provides the means to derive the speciation and concentration of species in multicomponent systems, and we have shown that the model-based measurement of the diffusion properties using Raman is a robust and accurate technique that allows for measuring the individual diffusion coefficients of the species in the mentioned system.