Polymerization reactions are generally risky reactions for two reasons. On the one hand, they are strongly exothermic reactions; and on the other hand. the viscosity of the reaction masses can strongly increase during the course of the reaction. Especially in homogeneous systems, the increase of viscosity causes a sharp decrease of the heat transfer coefficient. Moreover, there are several polymerization systems which show strongly autocatalytic behavior at higher polymer volume fractions. In the case of loss of agitation or cooling, it is necessary to bring the reaction back to safe operating conditions. The addition of a so-called stopping agent is one of the possibilities to prevent a reaction system from runaway under such circumstances. These stoppers are radical-trapping agents which can react with free radicals by formation of a terminated product with respect to the polymerization process. With respect to the free radical, inhibition and chain growth are the two reactions which compete with each other, According to the reaction rate ratio of these two reactions, these agents are classified into retarders and true inhibitors. In the case of an inhibitor the rate of inhibition is much faster than the rate of chain growth, in the case of a retarder both reaction rates are of nearly the same order of magnitude. Surveys of the literature with respect to the inhibition of polymerization reactions are given in [1,3]. The present paper deals with the inhibition of radical solution and suspension polymerizations of methyl methacrylate using 4-hydroxy-2,2,6,6-tetramethylpiperidinoxyl in an adiabatic reaction calorimeter.