A simple model describes the translational diffusion origin of preferential solvation. Assuming independentpolar ligands in the first solvation shell of the excited probe molecule, it presents an extension of theSmoluchowski aggregation model to the reversible case. The model captures the main aspects of bothconcentration and time dependence of spectral shifts obtained from steady-state and transient fluorescencemeasurements. It provides a useful method for extracting the rate and equilibria coefficients for ligand exchangein the first solvation shell of excited polar molecules.