The ring opening of 1,3-di-tert-butylaziridinone by tert-butylamine and aniline was investigated by using electrospray ionization and collision-induced dissociation in an ion trap mass spectrometer in conjunction with (15)N labeling of the two amine nucleophiles. Using the MS(n) capabilities of the ion trap instrument, we were able to monitor the retention of the (15)N label through successive fragmentation steps. Both amines exhibited a remarkable degree of selectivity in that they both cleaved exclusively the 1,3-bond (the alkyl-nitrogen bond). This result is in contrast to that obtained previously with methylamine, which cleaved just the opposite bond, namely, the 1,2-bond (the acyl-nitrogen bond). These contrasting results could not have been predicted by previously published guidelines.