In far u.v. photochemistry, the radicals formed react with hydrogen sulfide to give, amongst others, thiyl radicals (HS•). These radicals initiate the cis–trans isomerization of 2-olefins in a chain mechanism. We have tried to demonstrate that the thiyl radicals are also chain carriers. Quantum yields for the isomerization of cis to trans compound are probably of the order of 6–7 × 103 and the length of the chain, of the order of 4.7 × 103. We have also studied the radiolysis of these reactions in the liquid phase. The effect of oxygen and of conjugated di-olefins, such as 1,3-butadiene and 1,3-pentadiene is also discussed.