We present data on the number of active centers (CP) and the propagation rate constant (kP) upon copolymerization of ethylene and 1-hexene over supported titanium–magnesium catalysts of different compositions obtained by polymerization quenching with radioactive carbon monoxide (14CO). The rise in ethylene/1-hexene copolymerization rate as compared to the rate of homopolymerization (the comonomer effect) is found to be related to the increase in the calculated propagation rate constant. Meanwhile, the increased 1-hexene concentration results in a higher calculated kP value. Addition of 1-hexene during ethylene polymerization also increases the polymerization rate by raising the calculated kP value. These findings demonstrate that the comonomer effect is caused by the increase in monomer concentration in the copolymer layer that is formed on a catalyst particle and is characterized by reduced crystallinity as compared to that of homopolymer. The decline in the copolymerization rate with time is caused by reduction of the number of active centers.