In the photochemical bromination of 2-butene-1,4-diol, its diacetate, and 2,5-dimethyl-3-hexene-2,5-diol in nonpolar solvents, trans-addition products are formed, whereas their acetylenic analogs give mainly cis-dibromides. The differences in the behavior of acetylenes and the corresponding olefins are to be explained on the view that under identical conditions they may react by different mechanisms. Thus, under given conditions 2-butyne-1,4-diol diacetate is brominated by a chain mechanism, whereas the corresponding olefin is brominated by a nonchain mechanism.