The depression of glass transition temperature in polystyrene and styrene/divinylbenzene networks, by the addition of ethylbenzene, ethyl acetate, and m-diethylbenzene, respectively, has been investigated by differential scanning calorimetry. The predictive quality of the Couchman-Karasz treatment for the composition dependence of T, in compatible systems was found to provide excellent agreement with experimental results for the depression of Tg by ethylbenzene, provided certain modifications were made. Qualitatively, the theory predicts that for a given diluent, the depression of Tg is particularly sensitive to the incremental change in heat capacity of the pure polymer or network. The behavior of ethylbenzene provided good quantitative agreement with the theory, provided that the incremental change in heat capacity of the diluent a t its Tg was approximated by AC, = constant/T and that the incremental change in heat capacity of the network a t T,: represents only those units capable of thermal activation. The anomalous behavior observed on plasticization by ethyl acetate and m-diethylbenzene could in part be explained by a reduction in solvent quality or syneresis of diluent in the highly crosslinked samples.