The cationic photopolymerization of a biscycloaliphatic and a di-glycidyl epoxide monomer was investigated in the presence of an epoxy hydroxy functionalized polybutadiene (PBE). Triphenylsulfonium hexafluoroantimonate was used as photoinitiator. The kinetics of UV curing, in the presence of the additive, was determined via FT-IR analysis, showing an increase of the rate of photopolymerization and of the final epoxy groups conversion. Comparison experiments were performed with hydroxy functionalized polybutadiene. Evidences are obtained that PBE is incorporated via a copolymerization reaction involving the epoxy groups and via a chain transfer reaction involving the OH groups. The investigation of the properties reveals a flexibilization effect due to the presence of the additive, evidenced by a sharp decrease of T g of the cured films.