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, Δδ = δsolvent - δPS, in the values of the Hildebrand parameter (δ) between the two components of the binary systems: solvents having values of δ similar to those for PS (small Δδ) exhibit fast diffusion into the polymer matrix, whereas the diffusion slows down considerably when the δ's are different (large Δδ). 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.