Fourier transform infrared spectroscopy was used to detect changes or degradation in chain structure of PC/ABS specimens after being exposed to three hygrothermal conditions. The two main absorbance peaks of interest are associated with carbonyl and hydroxyl groups due to the oxidation of butadiene in the ABS component. The changes to these groups were evident in earlier work during thermal aging of this blend but were not found in this hygrothermal study. However, the increase in intensity within the hydroxyl region for two of the conditions was attributed to bulk water diffusion in the material. When viewed together with results published earlier by our research group, the changes driven by bulk moisture diffusion are reversible. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41316.