AbstractTwenty‐four pure fragrance ingredients of concern as potential skin sensitizers were previously subjected to degradation studies and evaluated using the high throughput with dansyl cysteamine (HTS‐DCYA) method. The experimental results showed that two‐thirds of the 24 fragrance ingredients underwent chemical degradation. In some cases, such degradation was accompanied by an increase in thio‐reactivity. These results prompted us to investigate the reactivity of the same ingredients using the direct peptide reactivity assay (DPRA). In the present work, the 24 chemicals were subjected to forced degradation for 150 days, and evaluated with both DPRA and HTS‐DCYA methods. At the end of the study, four and eight compounds remained non‐reactive in the DPRA and DCYA assay, respectively. Coumarin, benzyl salicylate, benzyl cinnamate and hexyl cinnamal were found unreactive in both assays, while cinnamal, cinnamyl alcohol, hydroxycitronellal and lilial were found negative in the DCYA but positive in the DPRA method. The incongruity in reactivity of these four compounds was attributed to a possible role of pro‐oxidants formed upon degradation, resulting in depletion of peptide without formation of apparent covalent adducts with the test chemical. To validate this hypothesis, the effect of hydrogen peroxide as model pro‐oxidant on both lysine‐ and cysteine‐heptapeptide depletion in the DPRA method was thus investigated. The obtained results showed little effect of oxidative conditions on lysine depletion, while cysteine depletion was significantly affected by concentrations above 1.1 mg/L of hydrogen peroxide. Overall, both in chemico methods confirmed chemical instability should be considered when assessing the skin sensitization potential of (un)known chemicals with alternative methods.