The curing behaviour of bismethacryloxy derivative of diglycidyl ether of bisphenol A (vinyl ester resin) containing styrene as the reactive diluent (40% w/w) was studied using gel point determination method and DSC. Seven samples of styrene/α-methylstyrene in the ratio 40∶0, 35∶5, 30∶10, 25∶15, 20∶20, 15∶25 and 0∶40 were studied. Delayed curing was observed in samples containing increasing proportions of α-methylstyrene. The energy of activation decreased from 869 kJ mol−1 to 333 kJ mol−1 as the concentration of α-methylstyrene increased in the formulations. However, no difference in thermal stability was observed by replacement of styrene by α-methylstyrene. It was concluded that in vinyl ester resin samples 10–15% α-methylstyrene and 30-25% styrene can be used as reactive diluent.