γ,γ‐Dimethylallenyl 1‐alkynyl sulfides 8a–d have been prepared by reaction of the corresponding alkynyllithium with γ,γ‐dimethylallenyl thiocyanate while the selenide analogs 10a,b have been obtained by an SN2′ reaction of the appropriate alkynyl selenide with α,α‐dimethylpropargyl bromide. Sulfides 8a–d display a high thermal reactivity, undergoing cyclization to the corresponding thiophenic derivatives 11a–d, together with thiophenic dimers such as 12–15, and some polymerization, under certain conditions. In contrast, selenides 10a,b are generally unreactive. The reaction exhibits a remarkable solvent effect. In isopropanol, only monomeric products are observed and the yield is greatly improved. An intramolecular free radical mechanism, involving the diradical intermediate 19, is suggested on the basis of the observed dimerization, solvent effect, deuterium labeling and a kinetic study of the reaction. The differences in reactivities between the analogous sulfides 8 and selenides 10 can be attributed to the varying ability of the respective heteroatom to stabilize the vinyl radical intermediate.