Poly(styrene-co-methacrylonitrile)s were polymerized in solutions with different polarities (n-hexane and THF) by low-temperature gamma-ray irradiation polymerization in a temperature range of -83.6-30 degrees C. It was found by IR measurement that the composition of the copolymers changed remarkably due to the effects of the polarity of solvents and the polymerization temperature. The thermal degradation behavior in the flash pyrolysis and in the continuous heating pyrolysis of these copolymers was measured by Py-GC and controlled rate thermogravimetry (CRTG). The effects of the copolymer composition and sequence distribution on the thermal degradation behavior were investigated.