Abstract This work investigates the accelerated thermal degradation of acrylonitrile–butadiene–styrene (ABS) due to aging at elevated temperatures (>80 °C). The impact resistance is shown to decrease dramatically beyond a critical aging time at 120 °C and this reduction strongly depends on surface property modifications during aging. Visual examination of specimen cross-sections after aging, verifies that (dis)colouration is limited to a surface layer, which is characteristic of degradation where oxygen diffusion into the bulk is limited. Degradation is supported by chemiluminescence assessment, which shows a rapid depletion of residual stabiliser within this layer as compared to the bulk polymer. Micro-indentation measurements also indicate that degradation causes an increase in Young's modulus at the specimen surface, which in turn promotes brittle failure. It is proposed that a critical depth of degradation (approximately 0.08 mm) forms on the surface of ABS due to aging. Applied loading initiates microcracks in this degraded layer, which propagate rapidly, causing bulk failure. Absorbance bands from Fourier transform infra-red spectroscopy indicate that surface degradation proceeds by chain scission and cross-linking in the polybutadiene (PB) phase of aged ABS specimens. Cross-linking is also supported by positron annihilation lifetime spectroscopy, which shows a decrease in free volume sites at the surface of aged specimens. Dynamic mechanical thermal analysis also supports the occurrence of cross-linking, as shown by an increase in the glass transition temperature of the PB phase after aging. Although degradation in the styrene–acrylonitrile (SAN) phase is less significant to the reduction in overall mechanical properties of ABS compared to the PB phase, an assessment of SAN copolymer indicates that heat aging decreases impact resistance. The contribution of SAN to the overall mechanical properties of ABS is also reflected by aging ABS specimens at temperatures just below the glass transition of the SAN phase (∼112 °C). The mechanism of thermal degradation is shown to be non-Arrhenius and governed by diffusion-limited oxidation. The long-term impact strength of ABS at ambient temperature is extrapolated from short-term data at elevated temperatures. As temperature and aging time influence degradation, it is proposed that at ambient service temperatures (40 °C), the degradation mechanism differs to that at elevated temperatures, and comprises both surface and bulk polymer degradation effects.