Polyvinyl chloride resin is softened by certain liquids, called plasticizers, through a set of processes with high activation energies, 50–100 kcal. At fixed temperature, ``reaction'' rates are directly proportional to vapor pressure of the liquid. The dielectric change may be treated similarly to a chemical reaction between a solid and a vapor, and free energy of activation, &Dgr;F**, evaluated. &Dgr;F** is constant for various liquids with one resin. &Dgr;E**=&Dgr;H**+Lin this system, where &Dgr;E** is Arrhenius activation energy, &Dgr;H** energy barrier,Llatent heat of vaporization of the liquid.