A flow mixing calorimeter has been used to measure the excess molar enthalpy H-m(E) of gaseous (ammonia + benzene) and (ammonia + cyclohexane) at the mole fraction y = 0.5, at standard atmospheric pressure. and over the temperature range 363.15 K to 493.15 K. Second virial coefficients B and isothermal Joule-Thomson coefficients phi for benzene and cyclohexane were fitted by the Kihara potential, and similar properties for ammonia were fitted by the Stockmayer potential. Cross terms B-12 and phi(12) were calculated using the arithmetic mean rule for collision diameters and the rule epsilon(12) = (1 - k(12))(epsilon(11) . epsilon(22))(1/2) for the depth of the potential well. The H-m(E) measurements on (ammonia + cyclohexane) were fitted to within experimental error by the choice (1 - k(12)) = 0.92. At the m temperatures 363.15 K and 493.15 K the values of H-m(E) for (ammonia + benzene) were found to be 9 J . mol(-1) and 5 J . mol(-1) less positive than the values calculated using (1 - k(12)) = 0.92. This difference was attributed to a specific interaction between ammonia and benzene. The difference was analysed in terms of a quasi-chemical association model in which the second virial cross coefficient B-12 was written B-12 = Bn-12(ns) -(RTK12)/2. The non-specific term B-12(ns) was calculated using (1 - k(12)) = 0.92, and values of the 12 equilibrium constant K-12 were determined from the difference between the calculated and experimental excess enthalpies. A plot of ln K-12 against reciprocal temperature yielded the enthalpy of formation DeltaH(12) of the ammonia-benzene complex and this was found to be DeltaH(12) = -(4.6 +/- 2) kJ . mol(-1). The sum of the specific and non specific contributions is - (8.5 +/- 3) kJ . mol(-1), and this is in reasonable agreement with values of the binding energy of the ammonia-benzene van der Waals complex computed by ab initio methods.