AbstractRadicals formed in γ‐irradiated 1,3,5‐trithiane and its derivatives at room temperature have been studied by ESR spectroscopy. ESR evidence establishes that 1,3,5‐trithiane as well as α‐ and β‐2,4,6‐trimethyl‐1,3,5‐trithiane yield radicals mainly by loss of the hydrogen atom, whereas α‐ and β‐2,4,6‐triphenyl‐1,3,5‐trithiane and 2,4,6‐trimethyl‐2,4,6‐triphenyl‐1,3,5‐trithiane produce radicals that are parent radical cations, which are also present in small amounts in aliphatic trithianes. Furthermore, in all the spectra examined R–S.‐type radicals are observed. The probable initiation process of radiation‐induced 1,3,5‐trithiane polymerization is discussed briefly.