A flow mixing calorimeter has been used to measure the excess molar enthalpy H-m(E) of gaseous (ammonia + carbon dioxide) at the mole fraction y = 0.5, at standard atmospheric pressure, and over the temperature range 343.15 to 443.15 K. Second virial coefficients B and isothermal Joule-Thomson coefficients phi for ammonia were fitted by the Stockmayer potential, and similar properties for carbon dioxide were fitted by the Kihara potential. Cross terms B-12 and 012 were calculated using the arithmetic mean rule for the collision diameter and the rule epsilon(12) = (1 - k(12)) (epsilon(11) . epsilon(22))(1/2) for the depth of the potential well. Values of H-m(E) were predicted using the combining rule (1 - k(12)) = 2(sigma(11)(3) sigma(22)(3))(1/2) (sigma(12)(3))(I1I2)(1/2) (I-1 + I-2)(-1) which gave (1 - k(12)) = 0.94. Correction for dipole-induced dipole effects increased this value by 5 per cent to 0.98. At T = 353.15 K the calculated value of H-m(E) was 14J.mol(-1) but the experimental value was found to be only 1.5J.mol-. This, and the other experimental values of H-m(E), could all be fitted by (1 - k(12)) = 1.47. The difference between the calculated and experimental values of H-m(E) was analysed in terms of a quasi-chemical association model in which the second virial cross coefficient B-12 was written B-12 = B-12(ns) -(RTK12)/2. The non-specific term Bn-12(ns) was calculated using (1 - k(12)) = 0.98, and values of the equilibrium constant K-12 were determined from the difference between the calculated and experimental excess enthalpies. A plot of In K-12 against reciprocal temperature yielded the enthalpy of formation DeltaH(12) of the ammonia-carbon dioxide complex and this was found to be DeltaH(12) = -(8.6 +/- 2) kJ.mol(-1). The sum of the specific and non specific contributions is -(11.5 +/- 3) kJ.mol(-1).