Temperature oscillation calorimetry in laboratory scale stirred tank reactors has been applied to the free radical polymerization of acrylic monomers in solution. It has been shown that this method can be used to determine on-line during the reaction the heat transfer value of the reactor and simultaneously the rate of reaction. The information about the heat transfer value is contained in small, forced temperature oscillations. The experimental setup of the calorimeter used is simple and consists only of commercially available components. This new method can also be incorporated into existing laboratory scale stirred tank reactors.