Abstract Monocarboxylic acids with acyl functional group such as 5-oxohexanoic acid and 6-oxoheptanoic acid were characterized experimentally by electrospray ionization coupled to a triple quadrupole and TOF analyzer hybrid system. Collision-induced dissociation experiments at different activation energies were done to elucidate possible fragmentation pathways. These pathways were also studied on the theoretical level using DFT B3LYP/6-311++G(3df,3pd)//B3LYP/6-31+G(d) + ZPVE calculations. While all monocarboxylic acids fragment under loss of CO 2 and H 2O starting from their parent anion [ M − H] −, an unusual fragmentation behaviour could be observed in case of 5-oxohexanoic acid. Synchronous CO 2 and CH 2CH 2 elimination in a concerted mechanism was responsible for the lack of [ M − H–CO 2] − fragment ion. New anionic gas phase cyclization processes could be discerned in case of 5-oxohexanoic acid. Mechanistic differences in the fragmentation pathways of [ M − H] − anions formed from 5-oxohexanoic acid and 6-oxoheptanoic acid after deprotonation were investigated both experimentally and through DFT calculations. Successive water, ketene CH 2 C O, and H 2 eliminations were observed in the CID spectra of 6-oxoheptanoic acid parent anion. CO 2 ejection from the parent ion of 6-oxoheptanoic acid was inhibited due to the lack of stabilization for the formed fragment ion.