A new series of ruthenium(II) complexes with the general formula [RuII([12]aneS4)(NN)]X2, where NN are bidentate diimines, [12]aneS4 is 1,4,7,10-tetrathiacyclododecane, and X is Cl− or PF6−, were studied by electrospray mass spectrometry (ESMS). Thiacrown fragmentation and intact diimine loss are the two major types of gas-phase reactions observed. The diimine losses occur mainly from the ion pairs formed by the inner sphere cations with halogen anions and are more significant for the complexes with flexible cross-bridged diimines. The fragmentations occur predominantly without changes in the oxidation state of the metal centre.