The triangulo Re ₃ ⁹⁺ cluster [Re ₃ (µ-Cl) ₃ X ₉ ] ^3- (X = Cl 1a or Br 1b) in concentrated aqueous hydrohalogenic acid solutions underwent a facile one-electron reduction by various reducing agents (VCl ₂ ·4H ₂ O, Sn–SnCl ₂ or Hg) to give the air-sensitive Re ₃ ⁸⁺ anion [Re ₃ (µ-Cl) ₃ X _9-n (H ₂ O) _n ] ^{(4-n) -} (X = Cl 2a or Br 2b), where n may be 1; the diamagnetism and EPR silence of this compound indicate that it may exist as a dimer with a direct or indirect rhenium–rhenium bond linking two Re ₃ ⁸⁺ trimetal cores. Oxidation of 2 by molecular oxygen in 6 mol dm ^-3 HX (X = Cl or Br) solutions yielded 1 quantitatively, whereas in the absence of acid and in aprotic solvents oxo-derivatives are formed; from 2b the hexanuclear anion [{Re ₃ (µ-Cl) ₃ Br ₆ (H ₂ O)(µ-O)} ₂ ] ^2- 3b was obtained whose structural characterization, as its [PPh ₄ ] ⁺ salt 3c, shows that two µ-O ligands bridge two oxidized Re ₃ ¹⁰⁺ units. The structure of the Re ₃ ⁹⁺ cluster [Co(en) ₃ ][Re ₃ (µ-Cl) ₃ Br ₈ (H ₂ O)]Br has been also determined by X-ray diffraction. The redox couple xRe ₃ ⁹⁺ –(Re ₃ ⁸ ) _x , where x = 1 or 2, derived upon mixing 6 mol dm ^-3 HX (X = Cl or Br) solutions of 1 with an excess of mercury, catalyses efficiently the reduction of molecular oxygen to water.