The degradation of acrylonitrile butadiene styrene (ABS) resin certified reference materials (CRMs) for heavy metal analysis was found. The masses of the ABS resin CRMs increased after the certification time, which resulted in decreasing the concentrations of elements. Other ABS resin CRMs contained 1 order of magnitude lower concentrations of same elements and showed no change for their concentrations and masses. The carbon and oxygen in degraded ABS resin CRM were ca. 4% lower and ca. 4% higher, respectively, than those of the nondegraded one. The increases for both ether (O-O-C) and ketone (C=O) groups and the decrease of C=C double bond were also observed for degraded ABS resin CRMs. From these results, it was considered that the oxidation occurred for the degraded ABS resin CRMs during the storage period. The Cr(III)-acetylacetonate in the degraded ABS resin CRMs was considered to be a metal catalyst to promote the oxidation.