A theoretical model was developed for describing the behavior of isothermal properties vs. the extent of cure for thermosetting glasses. The consideration follows the two-state Gibbs-DiMarzio (GD) theory of the glass transition. Network formation is considered on the basis of only interconnected trifunctional crosslinked sites using the Flory’s mean-held theory of gelation. Theoretically, the proposed model predicts that the fraction of hole volume (upsilon(0)) in the glassy state and the difference of expansion coefficient (Delta alpha) between glass and Liquid pass through a minimum with increasing extent of cure for thermosetting materials. These results suggest that the isothermal density and modulus will pass through a maximum with increasing extent of cure. Experimental data support the theoretical results. A discussion of the mechanism is included.