The use of isoperibolic (near-isothermal) and adiabatic reaction calorimetry is discussed with particular reference to a chlorination reaction. The reaction, involving the semi-batch addition of a liquid substrate to a chlorine-saturated solution of dichloromethane containing a catalytic quantity of pyridine, was simulated under isoperibolic conditions (to assess the heat of reaction and extent of reagent accumulation at the proposed reaction temperature and with the maximum possible feed rate of substrate). This was followed by further adiabatic simulation of the reaction to assess the emergency relief size required to account for the worst case scenario (maximum substrate flow without vessel cooling). An assessment of the envelope of safe working conditions was then defined to allow continued safe manufacture.