Succinyl-CoA:3-hydroxy-3-methylglutarate coenzyme A transferase, previously identified in rat-liver mitochondria (Deana et al. (1981), Biochim. Biophys. Acta 662, 119-124), was purified to near homogeneity and further characterized. After the last purification steps consisting of Ultrogel AcA-44 filtration and agarose-hexane-coenzyme A chromatography, the enzyme was apparently tetrameric with a mass of 48-52 kDa determined by gel filtration on Sephadex G-75, ultracentrifugation through a sucrose gradient and SDS-gel electrophoresis. By means of a HPLC technique developed for measuring the CoA esters we could determine the enzyme activity in both forward and reverse directions and show that the kinetic constants, i.e., Km of reactants and Vmax, are not too different for the two reactions. Double-reciprocal plots of the enzyme velocities versus the concentration of one substrate at different fixed concentrations of the other substrate gave families of straight lines converging below the substrate-abscissa for both forward and backward reactions, indicating a kinetic mechanism of rapid equilibrium random Bi-Bi type. The competitive inhibition of the product succinate with respect to both reactants, 3-hydroxy-3-methylglutarate and succinyl-CoA, as well as the Haldane relationships are consistent with this conclusion. An inhibitory effect on CoA transferase activity by acetate, acetoacetate, acetyl-CoA, acetoacetyl-CoA, coenzyme A, carnitine, ZnCl2 and high concentrations of the monovalent anions ClO4-, F-, I- and Cl- was also found.