An equation, based on thermodynamic considerations to relate the glass transition temperature, T-g, to compositional variation of a polymer system, is adapted in this article for modeling the T-g vs. fractional conversion (x) relationship of reactive thermosetting systems. Agreement between the adapted equation and experimental T-g vs. x data is found for several thermosetting crosslinking systems (i.e., epoxies and cyanate ester/polycyanurate) as well as for reactive thermosetting linear polymer systems (i.e., polyamic acid and esters to polyimides). The equation models the experimentally obtained T-g vs. x behavior of thermosetting systems which include competing reactions. Agreement for widely varying molecular structures demonstrates the generality of the equation. The entire T-g vs. x relationship can be predicted for a thermosetting material by using the T-g vs. x equation and the values of the initial glass transition temperature, T-g0, the fully reacted system glass transition temperature, T-g infinity, and the ratio of the change in specific heat from the liquid or rubbery state to the glassy state (Delta C-p) at T-g0 and T-g infinity, Delta c(p infinity)/Delta c(g0). The values of T-g0, T-g infinity and Delta c(p infinity)/Delta c(p0) can be measured generally from two differential scanning calorimetric experiments.