New epoxynorbornane linseed oils (ENLOs) were prepared as a function of the norbornene content. The cationic photopolymerization of ENLOs was investigated with real-time infrared spectroscopy and photo-differential scanning calorimetry. The effects of reactive and nonreactive diluents on the polymerization rate were also studied. The diluents reduced the viscosity of the formulations, markedly accelerated the rate of polymerization of ENLOs, and increased their final conversions. The effects of the reactive diluent were compared for ENLOs and epoxidized linseed oil. The relative reactivity of the oxiranes was not as important as the viscosity of the reacting system, and it was proposed that the cationic photopolymerization of ENLOs was controlled by diffusion. (C) 2003 Wiley Periodicals, Inc.