Enthalpies of complex formation of fumarate, itaconate, maleate and succinate with calcium have been determined by direct calorimetry. By using the stability constants of the literature, Gibbs function and entropy were also obtained. The present data refer to the aqueous medium at T = 25°C and I = 0.5 mol l^?1. In spite of the trans-configuration of fumarate, examination of the present thermodynamic results indicates a probable bidentate behaviour for this ligand, as a consequence of a charge-transmission due to the carbon-carbon double bond. The lower stability of the fumarate complexes in comparison with those of its cis-isomer seems due to different solute-solvent interactions, while the metalligand bond strength seems similar in both cases. On the other hand the present results seem to indicate for the itaconate complexes a structure similar to that of maleate and succinate, without charge transmission.