Poly(acrylonitrile/butadiene/styrene) (ABS) is a two-phase material consisting of elastomer particles in a glassy polymer matric of styrene and acrylonitrile (SAN) [1]. The photooxidation of ABS was the subject of several studies [2, 3]. It was suggested that several processes will take place during photooxidation. These changes include the formation of hydroperoxide [4], chain breakage of the polystyrene, and the oxidation of the polymer as it was monitored by IR spectroscopy [4–6]. Also photooxidation affect the polybutadiene in ABS and oxidizes it, which results in the formation of hydroperoxide. There are no data available on the thermal degradation of ABS. In a previous study the thermal aging of recycled high-impact polystyrene was studied using UV-vis spectroscopy [7]. It was found that this method provided very useful information about the degradation of several industrial polymers [8–12]. In this paper thermal degradation of ABS is investigated by UV-vis and IR spectroscopy.