The effects of water addition on the cationic photopolymerizations of epoxycyclohexane monomers were investigated. Conversions detected by Raman spectroscopy and rate of polymerizations measured by Photo-DSC were notably increased as with water concentration up to 1.5 wt%. This suggests that water distinctly enhances reaction kinetics in two ways: (1) long-lived intermediates in the initiation step allow effective primary interaction between the intermediates and water and (2) the efficient proton release of epoxycyclohexane monomers can facilitate the successive additions of protonated monomer to the hydroxyl end group for chain transfer via activated monomer (AM) mechanism, thereby accelerating kinetics. To support rational evidence for chain transfer via AM mechanism, monofunctional methyl 3,4-epoxycyclohexane-1-carboxylate (MEC) monomer was used as a model monomer system. The number-averaged molecular weight (Mn), polydispersity indices (PDI) and active-center propagation lifetime were characterized. As water concentration increased, the Mn appreciably decreased along with the PDI, which approached unity. In addition, the active-center propagation lifetime increased with increasing water concentration to demonstrate that the addition of water results in cationic active center regeneration. These results conclusively provide evidence for the AM mechanism involving the cationic photopolymerizations of epoxycyclohexane monomers.