Volumetric data for gaseous ammonia–acetylene mixtures have been analyzed to obtain second virial coefficients as a function of temperature and composition. From the pure-component virial coefficients of ammonia and acetylene, Kihara-potential parameters were obtained and these were used to calculate the physical contributions to the second cross virial coefficients,B12. The difference between the experimentalB12and that calculated from the physical contribution is attributed to chemical interaction, or complex formation, between the two molecules. Chemical equilibrium constants have been calculated, and ΔH0and ΔS0for complex formation were found to be −(2 210) cal/mole and −16.6 cal/mole °K respectively. These results indicate very weak hydrogen bond formation between the nitrogen atom in the ammonia and the hydrogen atom in acetylene.