Addition of the stable radical, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (4-OH-TEMPO), to the reaction mixture in the photolysis of 7-silanorbornadiene in solution results in the decrease of the magnetic field effect detected for the yield of stable reaction product, tetraphenylnaphthalene (TPN). The dependence of TPN yield on 4-OH-TEMPO concentration obeys the Stern-Volmer equation; the estimated rate constant of the quenching of triplet dimethylsilylene by the stable radical is k(TEMPO) = 1.4 x 10(8) M-1. In accordance with the suggested hypothesis, the paramagnetic additives-the stable radical and O-2, which has been studied earlier-change the T-S conversion rate in the intermediate biradicals via the so-called spin catalysis mechanism.