Aliphatic and aromatic carboxylic acids with Ru3(CO)12 to give bridging carboxylate complexes containing the structural core unit [Ru(μ3-RCOO)(CO)2]2. The presence of an excess of acid results in side ligation of two carboxylic acid molecules, which are also hydrogen-bonded to the bridging carboxylate ligand; this structure was confirmed by X-ray crystallography. Most of the complexes readily form mon-aquato compounds one of which was investigated by X-ray crystallography. In the case of 2-methylbutyric acid a Ru tetrameter complex, in which two core units are bound by two RuORu bridges, was isolated; we suggest that this structural element constitutes the back-bone of the ruthenium carboxylate polymers that were obtained under the appropriate reaction conditions from the most of the acids. The intial poroduct of the reaction between Ru3(CO)12 and a carboxylic acid is [Ru(RCOO)(CO)3]2, which then reacts further. Its reactivity varies on going from straight chain to α-carbon branched chain aliphatic acids and to aromatic acids. Spectral and chemical evidence suggests an associative type mechanism for displacement of CO by other ligands in the primary product. The distribution and rates of formation of the products depend on the nature of the carboxylate ligand and the molar ratio of dodecacarbonyltriruthenium to acid and on whether the reaction is carried out in an open or a closed vessel.