But-3-enylthioimidoyl radicals were shown by EPR spectroscopy and end product analysis to ring-close predominantly in the 5-exo mode with a rate constant of 2.4× 104 s-1 at 300 K to afford substituted dihydrothiophenylmethyl radicals. This ring closure was in competition with dissociation to but-3-enyl radicals and an isothiocyanate. The dissociation predominated at temperatures above ca. 300 K.