Cationic photopolymerizations of multifunctional epoxides cure efficiently at room temperature. However, slow reactions relative to acrylates and brittleness associated with highly crosslinked networks limit applications. Here, photoinitiated cationic copolymerizations were investigated for systems containing terminal hydroxyl groups and two types of epoxides, namely cycloaliphatic diepoxide (EEC) and epoxidized elastomeric oligomers (EPOH). When the predicted bulk mixture viscosity increased five-fold over neat EEC due to added EPOH content, the initial EEC reactivity began to decrease. Overall enhancement in EEC polymerization rates and ultimate conversion were attributed to activated monomer mechanism associated with EPOH terminal hydroxyl groups. Real-time consumption of EPOH internal epoxides was successfully obtained from Raman spectra. Initial reactivity and ultimate conversion of the internal epoxides decreased with increasing EEC content, especially at low internal epoxide contents. These results indicate that reactivity of EPOH hydroxyl groups is higher toward EEC cationic active centers than those of EPOH during cationic photopolymerizations. (C) 2013 Elsevier Ltd. All rights reserved.