Alkenes and alkynes were brominated efficiently in high yields with (bromodimethyl)sulfonium bromide in acetonitrile at room temperature. Alkenes produced the corresponding trans-dibromo compounds while alkynes (except phenylacetylene) afforded both the trans-dibromo products (major) and the tetrabromo derivatives (minor). However, phenylacetylene furnished solely the tetrabromo compound.