A simple equation to model the dependence of the glass transition temperature (Tg) on the extent of reaction in highly cross-linked thermosetting polymers has been derived. The model assumes that the increase in Tg is caused by (a) decrease in chain-end concentration, (b) formation of effective cross-links, and (c) further decrease in the configurationl entropy due to departure from Gaussian behavior at high cross-link densities. Data of Tg as a function of conversion have been obtained for a multifunctional epoxynovolac system. This system undergoes a very sharp increase in Tg at high conversions, which can be adequately fit by the model developed here. It has also been found that the change in Tg at low conversions in the multifunctional system can be predicted from data obtained with a model monofunctional system. The equation derived here is compared to several equations published in the literature.