The diffusion of various solvents into a polystyrene (PS) matrix was probed experimentally by monitoring the temporal profiles of the Raman spectra and theoretically from molecular dynamics simulations. The simulation results assist in providing a fundamental, molecular-level connection between the mixing/dissolution processes and the difference, Delta delta = delta(solvent) - delta(PS), in the values of the Hildebrand parameter (delta) between the two components of the binary systems: solvents having values of delta similar to those for PS (small Delta delta) exhibit fast diffusion into the polymer matrix, whereas the diffusion slows down considerably when the delta s are different (large Delta delta). To this end, the Hildebrand parameter was identified as a useful descriptor that governs the process of mixing in polymer-solvent binary systems. The experiments also provide insight into further refinements of the models specific to non-Fickian diffusion phenomena that need to be used in the simulations.