The present paper deals with the mechanisms of photochlorination and thermal chlorination of polyethylene as influenced by the microtacticity evolution of the polymer under chlorination. The investigations have been carried out on a branched low-density polyethylene and a linear high-density polyethylene, both chlorinated under various conditions which enabled us to study the influences of the chlorination time, the chlorination temperature, the chlorination method, the solvents used, etc., on the mechanisms of chlorination. The high-resolution nuclear magnetic resonance analyses published elsewhere,5,6,12 the infrared spectroscopy,8 the ratios (Cl/C), and (number of CHCl units/number of CH2 units) and the degree of polymerization are the main results discussed.