The effect of introduction of the comonomer methylthiirane (MT) into the main chain of polyacrylonitrile (PAN) on the thermal transformations of the macromolecules in dilute DMF solution was studied. For this purpose, statistical copolymers of acrylonitrile and MT were synthetized and their pyrolysis in solution was investigated. It was found that introduction of the thioether group into the main chain of PAN results in essential changes in the thermal behaviour of the copolymers as compared to the homopolymer. The results of rheological, thermal and spectral studies on the solutions suggest that the direction of the thermochemical reaction changes on introduction of the thioether group into the PAN molecule. In such copolymers the reaction will preferably proceed by the -C≡N groups forming conjugated cyclic structures.