Substrate specificity of a dicarboxyl-CoA: dicarboxylic acids coenzyme A transferase purified from rat liver mitochondria was assayed. In addition to the previously identified substrates succinyl-CoA, 3-hydroxy-3-methylglutaryl-CoA and malonyl-CoA (Francesconi et al.(1989) Biochim. Biophys. Acta, 999, 163-170) also methylmalonyl-CoA, glutaryl-CoA and adipyl-CoA acted as enzyme substrates, with the latter thioester showing the highest apparent affinity. All corresponding dicarboxylic acids, but not oxaloacetic, citric, alpha-ketoglutaric, malic, fumaric and glutamic acids, acted as coenzyme A acceptor substrates. None of the tested monocarboxylic acids, or the corresponding coenzyme A esters, were enzymatically transformed by the here described coenzyme A transferase.