The redox system cumyl hydroperoxide (C)-sodium methylsulfite (A) is a very effective initiator for radical low-temperature polymerizations. In previous works a mechanism for the decomposition of this catalytic system was suggested, but not supported by experimental evidence; owing to the high decomposition rate of the catalyst the polymerization kinetics were followed using an adiabatic reactor working as a calorimeter. The experimental results give evidence for a bimolecular mechanism of reaction between C and A giving place to oxycumyl and sulfonic radicals. The rate constant of the bimolecular reaction resulted to be 0.85 mole−1 liter sec−1 at −30°C. The activation energy, measured in the range −20 to −50°C, was calculated as 8.3 kcal/mole, in agreement with the proposed decomposition mechanism.