Diffusion and sorption of a variety of organic liquids into six engineering polymers have been investigated over the interval of 25°–60°C by the use of a conventional weight-gain experiment. While the method is laborious and time consuming, it yields reliable diffusion data. The transport mechanism depends, to a great extent, on the type of the solvent molecule and the barrier material. In all the polymer-solvent systems, the activated diffusion mechanism is operative and that the Arrhenius parameters of diffusion prove the conventional wisdom that smaller diffusion coefficients and higher activation energies are observed for the bigger solvent molecules. The diffusion mechanism has been classified to be anomalous. Sorption equilibrium data have been used to estimate the polymer-solvent interaction parameter in addition to molar mass between crosslinks and other thermodynamic quantities such as enthalpy and entropy of sorption. A first-order kinetic model appears to explain satisfactorily the transport of solvent molecules into polymer networks. Furthermore, attempts have been made to generalize, at least in qualitative terms, the polymer-solvent interactions.