The end groups of styrene-methyl methacrylate (St-MMA) copolymers polymerized radically with 2,2'-azobisisobutyronitrile (AIBN) as an initiator, which are difficult to characterize even by NMR, were investigated by pyrolysis-gas chromatography. On the resulting pyrograms, characteristic products that formed from the end-group moiety due to AIBN, such as 2-cyanopropane, 2-cyanopropen, and various compounds consisting of an isobutyronitrile group and a monomer unit, were observed together with those from the main chain, such as St and MMA monomers and various dimeric and trimeric products. The relative abundance between the recombination and disproportionation termination reactions in the copolymerization process was estimated from the relative intensities between the characteristic peaks of the end group and those of the main chain. Thus, the estimated abundance for the termination reactions suggested that the polymerization process for this particular copolymer system terminated preferentially by recombination rather than by disproportionation. Furthermore, the relative abundance between the monomer units adjacent to the chain-end AIBN residues was estimated on the basis of the peak intensities of the products consisting of an isobutyronitrile group and either monomer unit, which reflected the penultimate neighboring structure of the end group in the polymer chain. Thus, the observed results suggested that the isobutyronitrile radical formed by the dissociation of AIBN in the initiation reaction was predominantly adjoined by St monomer rather than by MMA monomer.