The diffusion of a fluorinated hydrocarbon into glassy poly(styrene) (PS) was studied by using resonance nuclear reaction analysis. It was found that; (1) the activation energy derived from measured diffusion coefficients is in good agreement with the value calculated from measured induction times, (2) the diffusion coefficient depends on molecular weight of PS, reflecting the dependence of the mobility on the polymer chain matrix, (3) the mathematical foundation of Lasky and Kramer provides a framework to obtain swelling kinetics from the experimental diffusion measurements.