The compounds [Ni(acac)(2)(L)(2)] (where acac is acetylacetonate and L is aniline (an), 2,6-dimethylaniline (dimean), o- or m-chloroaniline (o- or m-clan)) were synthesized and characterized by melting points, elemental analysis, TG-DTG, and DSC curves and electronic and IR spectroscopy. The enthalpies of dissolution of the adducts, Ni(II) acetylacetonate and ligands in a 1 : 3 (v/v) mixture of aqueous HCl 1.2 M and methanol, were measured and by using thermochemical cycles the following thermochemical parameters for the adducts have been determined: The standard enthalpies for the Lewis acid/base reaction (Delta(r)H(theta)), the standard enthalpies of formation (Delta(f)H(theta)), the standard enthalpies of decomposition (Delta(D)H(theta)), the lattice standard enthalpies (Delta(M)H(theta)) and the standard enthalpies of the Lewis acid/base reactions ire the gaseous phase (Delta(r)H(theta)(g)). The mean standard enthalpies of the nickel-nitrogen bonds have been estimated.