An alternative method to obtain the product of the overall heat transfer coefficient by the transfer area (U·A) in heat flow reaction calorimetry under reflux conditions is exposed. The new method uses the dynamics of the system during non-steady states to obtain U·A and the effective value of the heat capacity flow (ṁc·cPc) of the cooling fluid in the condenser. The validation of this method, its statistical evaluation and its limitations are extensively treated using pure solvents and water at their boiling point. The dynamic method gives excellent estimates of U·A, being less dependent on stirring speed and geometry of the reactor cover than the pseudo-dynamic method. The probability of obtaining an error smaller than 10% is higher than 95%, except for small volumes and very high stirring speeds.