AbstractTwo new techniques for the determination of monomer reactivity ratios in copolymerization under high‐pressure conditions have been developed, viz., the “sandwich” and the “quenching” method. Both methods are based on repeated quantitative gas chromatographic analysis of the reaction mixture during the low‐pressure stages preceding and succeeding the high‐pressure stage, of which the kinetics is under investigation. Application of the “sandwich” method implies the occurrence of reaction during both low‐pressure stages and consequently the low‐pressure kinetic data are required to obtain the transition points of low to high pressure and vice versa. These points constitute the initial and final conditions of the relevant high‐pressure reaction. On the contrary, in the “quenching” method no reaction occurs during the low‐pressure stages, owing to the lower temperature and the high activation energy of the initiator decomposition. As a consequence, the initial and final conditions of the high‐pressure stage can be determined by a simple averaging procedure. Both methods have been tested for the ethylene—vinyl acetate copolymerization at 62°C and 600 kg/cm2 with tert‐butyl alcohol as solvent, and appear to lead to almost identical monomer reactivity ratios, although the “quenching” method is slightly preferred in case of copolymerization reactions. Both methods are particularly valuable when one of the reactants is gaseous or the reaction produces a gas. Further merits and drawbacks of both methods are discussed.