The visible-light photchomopolymerization reactivities of several monofunctional oxiranes were evaluated using photodifferential scanning calorimetry (PDSC). Two oxiranes, styrene oxide and 1-methoxy-2-methyl propylene oxide, were selected for copolymerization reactivity studies with five substituted 1,5,7,11-tetraoxaspiro[5.5]undecanes (TOSUs). Reaction mixtures contained a diaryliodonium salt photoacid initiator and a beta-diketone photosensitizer. Experimentally determined reaction enthalpies were compared with calculated theoretical values to assess percent conversion. Relative reactivities were evaluated by comparing induction and exotherm peak maximum times. Results of AM I semiempirical quantum mechanical calculations of reaction energetics were compared to experimental findings for selected polymerizations. IR spectral changes were consistent with oxirane and TOSU ring opening. The effect of temperature on the photopolymerization reactivity characteristics of glycidyl methylphenyl ether alone and in combination with unsubstituted TOSU was also studied.