The initiation mechanism for spontaneous copolymerizations of vinyl sulfides (VS) with electron-accepting monomers such as maleic anhydride (MAn), diethyl fumarate (DEF), acrylonitrile (AN), and methyl acrylate (MA) was investigated by means of spin trapping technique using 2-methyl-2-nitrosopropane as a spin trapping agent. From the ESR spectrum observed from the system ethyl VS-DEF, two types of radicals, a vinyl radical (I; RSCH=CH) and an alkyl (1, 2-dicarboethoxyethyl) radical (II; C2H5OCOCHCH2COOC2H5) which derived from VS and DEF, respectively, were detected as their nitroxides. Similar radicals, I and III (NCCH2CH2), were also observed from the system VS-AN, but in the system VS-MA, three types of radicals, I, IV (CH3OCOCH2CH2), and V (CH3OCOCHCH3) were trapped as their nitroxides. In the system isopropyl VS-MAn, I and a propagating radical (VI) ∼∼CH2CHSR, were detected. The system isobutyl vinyl ether-MAn also showed a weak ESR spectrum due to the nitroxide from Radical I. These initiating radicals were assumed to be produced from the charge transfer complex formed between both donor and acceptor monomers. Based on these results, the cross-initiation mechanism for spontaneous alternating copolymerization is discussed.