Temperature Oscillation Calorimetry (TOC) is a technique for the on line determination of heat transfer coefficient UA, and is therefore suitable for studying processes where there is a large increase in the viscosity and a consequent decrease in UA as the reaction proceeds. The free radical polymerization of methyl methacrylate in ethyl acetate has been studied in a stirred, jacketed calorimeter with a volume of two litres. Sinusoidal temperature oscillation of amplitude 1-2°C and period 5-6 minutes are imposed on the fluid circulating in the jacket. The heat transfer coefficient is obtained from an analysis of the oscillating parts of the jacket and reactor temperatures, while the chemical heat flow is obtained from the non-oscillating parts of these temperatures. The results obtained show the utility of TOC in studying polymerization processes. A limitation has been encountered with concentrated solution where both the temperature oscillation and the value of UA may become irregular. Under these conditions a correlation between UA and the stirred torque may be used to correct the values of UA.