The effect of hydrogen on the number of active centers (СР) and the propagation rate constant (kP) at ethylene polymerization over titanium–magnesium catalysts is studied using quenching by radioactive carbon monoxide (14СО). The polymerization rate drops after hydrogen was added to the polymerization medium mainly due to the decrease in the calculated kP value. The changes in the polymerization rate and the experimental kP value observed after hydrogen was added or removed are found to be reversible. The data on the effect of the [H2]/[C2H4] molar ratio in the gas phase within the range of 0.125–1 on polymerization rate and the СР and kP values are obtained. Reduction of the kP value and a small decrease in the number of active centers are observed at the H2/C2H4 molar ratio ≥ 0.5. These findings were used to propose a scheme interpreting the effect of hydrogen on the СР and kP values. The scheme suggests that the decrease in the calculated kP value observed in the presence of hydrogen is related to the reversible formation of temporarily inactive dormant sites containing the titanium–polymer bond.