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Rationale: Although monoketone curcuminoids (MKCs) have been largely investigated due to their biological activities, data on the gas‐phase fragmentation reactions of protonated MKCs under collision‐induced dissociation (CID) conditions are still scarce. Here, we combined electrospray ionization tandem mass spectrometry (ESI‐MS/MS) data, multiple‐stage mass spectrometry (MSn), deuterium exchange experiments, accurate‐mass data, and thermochemical data estimated by computational chemistry to elucidate and to rationalize the fragmentation pathways of eleven synthetic MKCs. Methods: The MKCs were synthesized by Claisen‐Schmidt condensation under basic (1–9) or acidic (10–11) conditions. ESI‐CID‐MS/MS analyses and deuterium‐exchange experiments were carried out on a triple quadrupole mass spectrometer. MSn analyses on an ion trap mass spectrometer helped to elucidate the fragmentation pathways. Accurate‐mass data and thermochemical data, obtained at the B3LYP/6–31+G(d,p) level of theory, were used to support the ion structures. Results: The most intense product ions were the benzyl ions ([C7H2R1R2R3R4R5]+) and the acylium ions ([M + H − C8H3R1R2R3R4R5]+), which originated directly from the precursor ion as a result of two competitive hydrogen rearrangements. Product ions [M + H – H2O]+ and [M + H − C6HR1R2R3R4R5]+, which are formed after Nazarov cyclization, were also common to all the analyzed compounds. In addition, •Br and •Cl eliminations were diagnostic for the presence of these halogen atoms at the aromatic ring, whereas •CH3 eliminations were useful to identify the methyl and methoxy groups attached to this same ring. Nazarov cyclization in the gas phase occurred for all the investigated MKCs and did not depend on the presence of the hydroxyl group at the aromatic ring. However, the presence and the position of a hydroxyl group at the aromatic rings played a key role in the Nazarov cyclization mechanism. Conclusions: Our results reinforce some aspects of the fragmentation pathways previously published for 1,5‐bis‐(2‐methoxyphenyl)‐1,4‐pentadien‐3‐one and 1,5‐bis‐(2‐hydroxyphenyl)‐1,4‐pentadien‐3‐one. The alternative fragmentation mechanism proposed herein can explain the fragmentation of a wider diversity of monoketone curcuminoids. [ABSTRACT FROM AUTHOR]