The relative photoreactivity of five expanding monomers (EMs) in homopolymerization, and as comonomers in a candidate low shrinkage dental matrix resin, were evaluated. The EMs were 2, 8-dimethyl-1,5,7,11-tetraoxaspiro[5.5]undecane (DM-TOSU); 3,9-diethyl-3,9-dipropionyloxymethyl-1,5,7,11-tetraoxaspiro[5.5]undecane (DEDPM-TOSU); 1,3-dioxane-2-one (DOO); 4-methyl-1,3-dioxane-2-one (M-DOO); and 5,5-diethyl-1,3-dioxane-2-thione (DE-DOT). The candidate low shrinkage resin system was an 80/20 mixture of UVR-6105 epoxide/polytetrahydrofuran (M n Ο· 250). All reaction mixtures contained a diaryliodonium salt as a photoinitiator and camphorquinone as photosensitizer. Reactivities were evaluated using photodifferential scanning calorimetry. For homopolymerizations, the reactivity ranking (based on time to exotherm peak and total enthalpy) was DE-DOT Σ· DM-TOSU ΟΎ DOO ΟΎ M-DOO Υ DEDPM-TOSU. In the comonomer system, the reactivity ranking was M-DOO ΟΎ DEDPM-TOSU ΟΎ DM-TOSU ΟΎ DOO Υ DE-DOT. This experimental work was substantiated and extended by molecular modeling studies employing the AM1 semiempirical method. Heats of formation of protonated EM structures, and heats of formation and potential energies of possible polymerization pathways were estimated. The relative reactivities of EM-based polymerization systems are related to chemical structure and the dominance of the most favored reaction mechanism.