A differential thermal analysis apparatus was constructed for the measurement of heats of polymerization. It featured an active cell in which the reacting mixture was contained in expendable thin-walled, narrow, cylindrical brass, or aluminum cartridges. The cartridges were tightly pressed into holes drilled into the body of the active cell and they, along with the solid reaction product, could be ejected therefrom at the completion of the reaction. A calibration cell was provided with small electric heaters which allowed known amounts of heat to be supplied to the apparatus in calibration runs. With this apparatus the mean value for three determinations of the heat of polymerization of epoxy resins, ca. 22 to 26 kcal./mole, could be measured to within a standard error of 0.37 kcal./mole (i.e., confidence limits of ±0.75 kcal./mole at the 95% confidence level). The heats of polymerization of phenyl glycidyl ether and of an epoxy resin were measured with ten and nine different curing agents, respectively. Primary amine curing agents released about 26 kcal./mole of epoxide reacted, tertiary amines, and boron trifluoride-ether complex each released about 22 kcal./mole. Mixed-type curing agents, which could react in part as tertiary amines, or boron trifluoride, and in part as primary amines could liberate intermediate amounts of heat.