The green complex formed on treating commercial ruthenium trichloride hydrate with acetic acid and sodium acetate in ethanol is shown to be of the basic acetate oxo-centred triangular type with the formula [Ru₃O(CO₂Me)₆-(H₂O)₃](CO₂Me). The complex may be cationic, neutral, or anionic depending on the pH of the solution, owing to ionisation of co-ordinated water. The water molecules may be replaced by pyridine to give [Ru₃O(CO₂Me)₆py₃]⁺. Both the aquo and pyridine complexes undergo successive one and two electron reductions to give complexes in which the metal atoms are in formal oxidation states + 2 ⅔ or (III, III, II) and +2 or (II, II, II) respectively. The reduction to the latter state involves loss of the central oxygen atom from the triangle of metal atoms; this atom may be reinserted using molecular oxygen, or more specifically by use of a reagent such as pyridine-N-oxide. In the presence of triphenylphosphine, [Ru₃O(CO₂Me)(H₂O)₃]⁺ gives the reduced species Ru₃O(CO₂Me)₆(PPh₃)₃. The potentials and reversibility of the redox reactions have been studied electrochemically. Analogous complexes of other carboxylic acids are described. N.m.r., i.r., and electronic absorption spectra of the species are reported.