A new combination of collisionally activated dissociation and ion-molecule reactions in a quadrupole ion trap mass spectrometer is developed for the class-selective determination of 4-quinolones, an important group of antibiotics. In this method, collisionally activated dissociation is used to promote dehydration of each protonated quinolone, and the resulting dehydrated fragment ion undergoes an addition reaction with an auxiliary reagent. The auxiliary reagents, including acetone, acetic anhydride, and n-propylamine, have structures that specifically allow formation of a cyclic adduct with the analyte ion. This sequential process is characteristic of a class of analyte molecules that have adjacent carboxylic acid and ketone functional groups anchored on a rigid ring and undergo facile, efficient dehydration.