Abstract: Photopolymerization of β-hydroxyethyl methacrylate (HEMA) under UVlight at 30℃ with the use of benzoin derivatives as the photoinitiators has been studied by a recording dilatometer. For each photoinitiator (hydroxy-methyl benzoin methyl ether or benzoin ethyl tther), in aqueous solution, the initial rates of polymerization (V_p) were determined as a function of (photoinitiator), (HEMA) and light intensity. The kinetic data showed that the dependence of rates on initiator and incident light intensity was approximate to usual free radical kinetics with square-root dependence, but the monomer exponent was much lower than unity in both cases. This deviation from normal kinetics has been interpreted as that bimolecular termination of chain radicals were interrupted by the poor solvent as polymerization media, especially when the diluent concentration was higher, so the polymerization rate was unusually higher and thus lowering the monomer exponent. Through kinetic studies, tentative mechanism of HEMA polymerization in H₂O was proposed. In addition, the effect of temperature and solvent on polymerization rate and post polymerization rate was also investigated. The overall activation energy of polymerization for the two systems were evaluated to be zero.