The reactivity ratios relative to cationic polymerization of three chlorostyrenes and styrene were studied. The values of r1andr2were determined for various experimental conditions. The influence of the solvent, of the polymerization temperature, of the nature of the initiator and of the molecular weight distribution were examined in particular. The relative activation entropies and enthalpies were determined (against styrene as reference) and an Isokinetic relationship was found for 2- and 3-chlorostyrenes. All the experimental reactivities were correlated with the quantum chemistry parameters. From this correlation it is found that the interaction is possible not only with C8, but also with C7 and CI, depending on the nature of the monomer.