The decomposition of di-cumyl peroxide (DCP) has been carried out in a commercial reaction calorimeter (ARSST(TM)), under near-adiabatic reaction conditions. The decomposition was then simulated by means of a simple model which used kinetics derived from concentration-time data obtained in isothermal experiments. This method allowed a more precise determination of the activation energy and pre-exponential factor for DCP decomposition. The model developed was able to predict with good accuracy the main characteristics the decomposition process, and was also used to investigate the sensitivity of the runaway with respect to the parameters values used in the simulation.