Electrospray ionization mass spectrometry (ESIMS) was used to characterize a series of new ruthenium(II) complexes with several nitrogen heterocycles and a common ligand: the crown thioether 1,4,7-trithiacyclononane, [9]aneS3. ESIMS allows the easy identification of the [RuIICl([9]aneS3)Y]X complexes, where Y is a bidentate nitrogen heterocycle and X is Cl− or PF6−, through the formation of two diagnostic ions by fragmentation of the common ligand [9]aneS3. Structures for these ions and mechanisms for their gas-phase formation are proposed based on data from product ion spectra. Copyright © 2001 John Wiley & Sons, Ltd.