Studies were performed on the free-radical thermal copolymerization of styrene and glycidyl methacrylate in a batch and a continuous stirred tank reactor. From experiments at low monomer conversions in a batch reactor the monomer reactivity ratios and their temperature dependence were determined between 110, 135, 160 °C according to the terminal model. The cumulative copolymer composition could be satisfactorily described as a function of monomer conversion with the equation of Meyer-Lowry. The values of the reactivity ratios could be successfully extrapolated to higher monomer conversions and higher reaction temperatures. The steady-state behavior of a continuous stirred tank reactor could be very well simulated with respect to the copolymer composition and to the residual monomer ratio up to a reaction temperature of 190 °C. At higher reaction temperatures the differences between theory and experimental results increased, perhaps due to the start of thermal decomposition of the copolymers.