Organic solvents penetrating into a spherical cross-linked polystyrene bead advance over a sharp front and cause the bead to swell. The changing location of the advancing front and the increasing radius of the swelling bead were measured by microscopic techniques. The rate of solvent uptake by the copolymer beads was shown to be of the Fickian type. A mathematical model of the Fickian diffusion process, showing both the advancing front and the expanding sphere, was developed. Effective diffusivity coefficients for methylene chloride, trichloroethylene, tetrahydrofuran, and benzene were obtained by least-squares fitting of the calculated and the measured times necessary for complete swelling.