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