In this chapter, we develop a model to describe the simultaneous diffusion and solvent induced crystal formation in polymers based on the idea that crystal formation is governed by polymer chain mobility in addition to a thermodynamic driving force. The polymer chain mobility is described using the free-volume theory of transport that has the ability to describe polymer mobility based on the type of solvent and polymer and their respective jumping unit sizes. The semi-crystalline polymer-solvent system is treated as a ternary system consisting of (1) crystalline polymer, (2) amorphous polymer and (3) solvent. The addition of solvent to the amorphous phase (amorphous polymer + solvent) is assumed to increase the local free volume and facilitate movement of polymer chains, thereby enabling crystal formation. Diffusion of the solvent is assumed to occur solely in the amorphous polymer phase since the crystalline regions of the polymer generally exclude solvent. The species continuity equations are formulated in volume-averaged coordinates, and a convective term arises in the analysis, due to the volume change accompanying transformation of the amorphous polymer to the crystalline polymer and also due to the movement of crystals. Accurate modeling of this problem requires that a moving boundary be considered. The model developed was tested using gravimetric sorption data for the polyvinyl alcohol-water system. In the experimental studies conducted, a high percentage of solvent was expelled from the thin samples during sorption experiment and the proposed model accurately predicts this behavior. When the sample contains a large amount of crystals or when the samples are sufficiently thick, classical Fickian behavior is observed.