AbstractStudies on the composition of copolymers obtained by the radiation‐induced solid‐state postpolymerization of trioxane with 1,3‐dioxolane have been carried out. Gas‐chromatographic analysis of the reaction mixtures showed that most of the 1,3‐dioxolane disappears rapidly from the reaction system in an early stage of polymerization, and that the fraction of ethylene oxide units in the copolymer chain [E] decreases markedly with increasing polymer yield. This finding was confirmed by NMR spectra of the copolymer. DSC thermograms of the copolymer indicated that the relationship between the melting point and the average composition of copolymers prepared in this study differed from that found for copolymers in which comonomer units are distributed statistically in the polymer chain. It was suggested that the copolymer formed by the radiation‐induced solid‐state postpolymerization of trioxane–1,3‐dioxolane is characterized by a heterogeneous distribution of ethylene oxide units in the copolymer chain. It was also found that, in the radiation‐induced solid‐state postpolymerization of trioxane–1,3‐dioxolane, the amount of tetraoxane formation increased linearly with increasing polymer yield. Although it is extremely small compared with that obtained in solution polymerization, it is slightly larger in the trioxane–1,3‐dioxolane syste