The triosmium cluster, [Os 3 (CO) 1 0 (CH 3 CN) 2 ], undergoes reaction with carboxylic acids to form clusters containing a chelating μ,η 2 -carboxylate ligand and a hydride bridge across one edge of the triangle via an intermediate species postulated, from IR and NMR studies, to contain a coordinated μ,η 1 -carboxylate ion. The structure of the cluster, [Os 3 H(CO) 1 0 (C 6 H 4 (OH)CO 2 )] (1), has been determined by single-crystal X-ray diffraction and contains the 2-hydroxobenzoate ligand bound via the carboxylate oxygens in the μ,η 2 -mode. A thiocarboxylate analogue of the postulated intermediate, viz. [Os 3 H(CO) 1 0 (C 6 H 5 COS)] (2), has been obtained and its structure determined by single crystal X-ray diffraction. The C 6 H 5 COS - ion, which is bound in the μ,η 1 -mode through the sulfur atom, bridges two of the osmium atoms of the triangle.