Calorimetric studies on the bulk polymerization of uninhibited styrene monomer under low temperature conditions from 50 to 85 degrees C were reported. Various thermograms acquired by either dynamic scanning or isothermal ageing were characterized by differential scanning calorimetry (DSC) or thermal activity monitor (TAM). Isothermal curve demonstrating an autocatalytic phenomenon associated with a characteristic induction time was detected. Heat of thermal polymerization of styrene between 50 and 85 degrees C was measured to be 670+/-11 J g(-1). The findings of this study include: (1) Exothermic phenomenon of thermal-initiation and chain transfer steps in thermal polymerization were corroborated by using calorimetric investigation extended to the temperature as low as 50 degrees C; (2) activation energies in the steps of thermal-initiation and chain transfer were determined to be 125+/-6 and 60+/-5 kJ mol(-1) by using Arrhenius plot, respectively; (3) apparent activation energy was verified to be 84 kJ mol(-1) related to the individual steps of thermal-initiation and chain transfer conformed. These kinetic parameters work well in simulating the adiabatic runaway behaviors of uninhibited styrene and in good agreement with other studies.