From solution calorimetry and appropriate thermodynamic cycles, the standard molar enthalpies of acid-base reactions in the condensed state (Delta(r)H degrees) for MX(2) . nCL (M is Zn, Cd, Hg; X is Cl, Br, I; n = 1, 2, 3 or 4; CL is epsilon-caprolactam) adducts have been determined. From Delta(r)H degrees values and literature data, the following thermochemical quantities have been calculated : standard enthalpy of formation (Delta(f)H degrees), standard enthalpy of decomposition (Delta(d)H degrees), standard lattice enthalpy (Delta(latt)H degrees), standard enthalpy of acid-base reaction in the gaseous phase (Delta(g)H degrees) and the mean dissociation enthalpy of the metal-oxygen bond, (D) over bar(M-O). The thermochemical data are compared with those previously published for analogous BuL compounds (BuL is gamma-butyrolactam or 2-pyrrolidone), with the purpose of evaluating the effect of ring strain on the donor oxygen strengths of both lactams. The thermochemical quantities are discussed in terms of acidity and basicity of the species and Delta(g)H degrees values correlated with ionic or covalent character of the metal halides.