This article deals with the determination of kinetic and thermodynamic data of free-radical polymerization by adiabatic reaction calorimetry. The polymerization of methyl methacrylate in solution, suspension, and emulsion were chosen as systems to be studied. From the measured temperature-time courses of the polymerizations the overall rate constants can be determined with and without gel effect. With knowledge of an appropriate mathematical model describing the kinetics of reaction it was also possible to estimate elementary reaction constants if the molecular weight distribution of the polymer formed was considered as well. The temperature rise and the self-heating rate can be modeled very well for polymerization in solution over the entire range of concentration and, in the case of polymerization in suspension and emulsion, up to a volume fraction of monomer of 20%. The modeling of molecular weight distribution of polymers produced by polymerization in solution and suspension is satisfactory. For emulsion polymerization, however, only the order of magnitude of the average molecular weight could be calculated with the model used. The average particle size of the polymer latex formed could be calculated rather well.