Structural information of the styrene-ethylene glycol dimethacrylate (St-EGDM) cross-linked copolymers obtained by Pyrolysis-gas chromatography (Py-GC) was compared with those obtained by conventional dynamic mechanical analysis and swelling measurement. The monomer ratios of St/EGDM in the highly cross-linked polymers determined by Py-GC were in fairly good agreement with the monomer feed ratios, while those estimated indirectly by the conventional methods were considerably higher than the feed ratios. Furthermore, the St contents estimated by Py-GC for the polymer samples obtained in low conversion were found to be extremely lower than the feed ratios suggesting that the cross-linking domains would be formed at the early stage of the polymerization. These observations suggest that the network structures of the cross-linked polymers in this work would be inhomogeneous to a great extent mainly due to the potential presence of the cross-linking domains in which EGDM monomer units are localized.