The effect of a constant magnetic field on the initiation of chain oxidation at 343 K in the presence of azo initiators increases with an increase in the magnetic field strength in the range of 0.3–0.6 T and does not depend on the nature of the oxidized compound and solvent. In the case of a peroxide initiator, no magnetic field effect is observed. For azo initiators, the probability of S–T+ and S–T− transitions in the singlet pair [r• •r] increases, thereby enhancing the efficiency of the release of radicals from the cell.