Disulfide polymers obtained by ring-opening polymerization have been considered to have a possibility of a cyclic catenane structure as judged from their test properties on the loss modulus and stress-strain. Electrospray ionization mass spectrometry (ESI-MS) was used to detemine molecular structure of polydithiane and polyoxyethylene to find whether they are present as a cyclic or as a linear structure. The results indicated that the polydithiane possesses the cyclic structure, and analysis of the isotope distribution of the spectral ions further showed that the polymer consists entirely of the cyclic structure without contamination of a linear polymer. A linear chain polymer with a benzylmercaptan end group was synthesized, and the ESI-MS analysis revealed that the polymer was a mixture of both the cyclic and the linear polymer. The cyclic polymer is probably formed by back-biting of the highly reactive sulfur radicals that had been formed during the polymerization reaction.