To understand the hapten-protein complex formation in the context of skin contact allergy to p-amino aromatic derivatives, 2,5-dimethyl-p-benzoquinonediimine was used as a model compound to study the reactivity of p-benzoquinonediimines, first oxidation intermediates of allergenic p-amino aromatic compounds, toward a model peptide containing naturally occurring and potential reactive amino acids. LC-MS analysis, together with electrospray ionization MS/MS, was used for the determination of amino acid selectivity by studying the chemical modifications induced on the peptide due to covalent binding of the p-benzoquinonediimine. Results reported in this paper indicated that 2,5-dimethyl-p-benzoquinonediimine reacted with the epsilon-NH(2) group of lysine to first form a covalent adduct of the Schiff's base kind. Besides, an oxido-reduction process started that induced an oxidative deamination of lysine to form a peptidyl alpha-aminoadipic-delta-semialdehyde, by a mechanism similar to the one known for several enzymatic quinonoid co-factors, followed by an intramolecular cyclization of the peptide. From these results it could be concluded that lysine must be considered as an important amino acid for the hapten-protein complex formation in the case of p-benzoquinonediimines and that, in addition to direct covalent binding, further degradation of the peptide can be produced.