Vapour—liquid equilibrium data of the acetylene—ammonia system for seven isotherms in the range −60 ÷ 25°C and 1 ÷ 32 atm are reported. Experimental values are interpreted according to the thermodynamics of the associated systems. It is shown that thermodynamic consistency is observed by assuming a bimolecular complex formation between acetylene and ammonia. Equilibrium constants for this complex in the liquid phase have been derived and related to the corresponding gas phase values known from the literature. Two models based on the association and the equations by Margules, Wilson and Van Laar have been employed for correlating and extrapolating the experimental values.