The trinuclear arene-ruthenium cluster cations [H 3 Ru 3 (C 6 H 6 )(C 6 H 2 Me 4 ) 2 (O)] + (2) and [H 3 Ru 3 {C 6 H 5 (CH 2 ) 2 OH}(C 6 H 2 Me 4 ) 2 (O)] + (3) have been synthesised from the dinuclear precursor [H 3 Ru 2 (C 6 H 2 Me 4 ) 2 ] + (1) and the mononuclear complexes [Ru(C 6 H 6 )(H 2 O) 3 ] 2 + and [Ru{C 6 H 5 (CH 2 ) 2 OH}(H 2 O) 3 ] 2 + , isolated and characterised as the tetrafluoroborate salts. Cations 2 and 3 are analogues of the cluster cation [H 3 Ru 3 (C 6 H 6 )(C 6 Me 6 ) 2 (O)] + which was found to catalyse the hydrogenation of benzene to give cyclohexane under biphasic conditions ('supramolecular cluster catalysis'). The single-crystal X-ray structure analyses of 2 and 3 have been determined. Unlike [2][BF 4 ].3H 2 O, [3][PF 6 ].H 2 O shows a dimeric structure in the solid state, thanks to hydrogen bonds between the hydroxo function of one molecule of 3, a water molecule and the oxo cap of an other molecule of 3.