Propolis is a resin-like material produced by bees fromthe budsof poplar and cone-bearing trees and is used in beehive construction.Propolis is a common additive in various biocosmetics and health-relatedproducts, despite the fact that it is a well-known cause of contactallergy. Caffeic acid and its esters have been the primary suspectsbehind the sensitization potency of propolis-induced contact allergy.However, the chemical structures of the protein adducts formed betweenthese haptens and skin proteins during the process of skin sensitizationremain unknown. In this study, the reactivity of three main contactallergens found in propolis, namely, caffeic acid (CA), caffeic acid1,1-dimethylallyl ester (CAAE), and caffeic acid phenethyl ester (CAPE),was investigated. These compounds were initially subjected to thekinetic direct peptide reactivity assay to categorize the sensitizationpotency of CA, CAAE, and CAPE, but the data obtained was deemed toounreliable to confidently classify their skin sensitization potentialbased on this assay alone. To further investigate the chemistry involvedin generating possible skin allergy-inducing protein adducts, modelpeptide reactions with CA, CAAE, and CAPE were conducted and analyzedvia liquid chromatography-high-resolution mass spectrometry.Reactions between CA, CAAE, and CAPE and a cysteine-containing peptidein the presence of oxygen, both in closed and open systems, were monitoredat specific time points. These studies revealed the formation of twodifferent adducts, one corresponding to thiol addition to the & alpha;,& beta;-unsaturatedcarbonyl region of the caffeic structure and the second correspondingto thiol addition to the catechol, after air oxidation to o-quinone.Observation of these peptide adducts classifies these compounds asprehaptens. Interestingly, no adduct formation was observed when thesame reactions were performed under oxygen-free conditions, highlightingthe importance of air oxidation processes in CA, CAAE, and CAPE adductformation. Additionally, through NMR analysis, we found that thioladdition occurs at the C-2 position in the aromatic ring of the CAderivatives. Our results emphasize the importance of air oxidationin the sensitization potency of propolis and shed light on the chemicalstructures of the resultant haptens which could trigger allergic reactionsin vivo.