Adducts formed between ZnX2 (X = Cl, Br, I) and 2-imidazolidinone (ethyleneurea, EU) or tetrahydro-2-pyrimidone (propyleneurea, PU) of the general formula ZnX2.2L (L = Eu or PU) had the standard molar enthalpies determined through reaction-solution calorimetry at 298.15 K: ZnX2(cr) + 2L(cr) = ZnX2.2L (cr); Delta(r)H(m)(0)/kJ mol(-1): ZnX2.2EU, -(46.03 +/-0.72); -(45.35 +/- 0.58) and -(59.20 +/- 0.83) for chloride, and iodide, and ZnX2.2PU, -(44.26 +/- 2.63); -(37.75 +/- 1.83) and -(46.96 +/- 1.43) for the same sequence of halides. From these values, the standard molar enthalpies of the crystalline adducts were calculated for both the series. The standard molar enthalpies of sublimation for EU and PU were calculated by means of differential scanning calorimetry to give 83.7 +/- 1.9 and 89.3 +/- 2.5 kJ mol(-1), respectively. These values enabled the calculation of the corresponding standard molar enthalpies of adducts in the gaseous phase. From these latter enthalpic values, the mean zinc-oxygen bond enthalpies were determined as 139.4 +/- 2.2, 130.1 +/- 2.2, 131.4 +/- 2.2 and 141.3 +/- 3.2, 129.1 +/- 3,0, 128.2 +/- 3.0 kJ mol(-1) for EU and PU adducts, for the above sequence of halides,