Magnetic resonance imaging (MRI) has been used to study the diffusion of 1,4-dioxane in poly(vinyl chloride) (PVC) and of acetone (or mixtures of acetone/methanol) in polycarbonate (PC). The results reveal that the system PVC/1,4-dioxane follows the classical Case II theory. The other two systems investigated (PC/acetone and PC with acetone/methanol mixtures) obey Fick’s law. In this case, a solvent-induced crystallization takes place after the diffusion front which, at the final stage, leads to a solvent-induced cracking of the material.