Abstract: A fast reacting, automated calorimeter includes a reaction vessel containing a chemically reacting mass, a fluid circulation system containing heat transfer fluid, a portion of the system passing the fluid through the reaction vessel for exchanging heat between the fluid and the reacting mass, a flow rate controller at least generally responsive to variations in temperature of the reacting mass to vary the flow rate of the fluid circulated through the reaction vessel portion of the circulation system, a flow rate signal generator generating a signal related to the varying flow rate of the fluid passing through the reaction vessel portion of the fluid circulation, and a circuit, preferably including a computer, responsive at least to the flow rate signal and generating a heat flow signal generally related to instantaneous rate of heat exchange between the reacting mass and the fluid.