Thermochemical data for adducts of N-methyl-2-pyrrolidone (NMP) with zinc(II), cadmium(II) and mercury (II) halides have been obtained from solution calorimetry. The technique provided the values for the dissolution enthalpies (Delta(i)H(0)) of MX2 (M = Zn, Cd, Hg; X = Cl, Br, I) metal halides, the NMP lactam and MX2 . nNMP adducts, which were used to determine the standard molar enthalpies of acid-base reaction in the condensed state (Delta(r)H(0)) for the adducts, by using appropriate thermodynamic cycles. From Delta(f)H(0) values and literature data, the following thermochemical parameters have been calculated : standard enthalpy of formation (Delta(f)H(0)), standard enthalpy of decomposition (Delta(d)H(0)), standard lattice enthalpy (Delta(latt)H(0)), standard enthalpy of acid-base reaction in the gaseous phase (Delta(g)H(0)) and the mean dissociation enthalpy of the metal-oxygen bond, (D) over bar (M-O) have been established. In order to evaluate the methyl group influence on the donor oxygen strength of the NMP lactam, the obtained thermochemical data are compared with those previously published for analogous 2-pyrrolydone (BuL) compounds. The influence of the ionic and covalent character of the metal halides has been taken into account in the metal-oxygen interaction processes.