Hydrofluorocarbons (HFCs) are known to have specific intermolecular interactions (hydrogen bonding) as well as large permanent electric dipole moments. The peculiar vapor-liquid equilibria (VLE) behavior (double azeotropes) observed in ammonia/R-125 mixtures led us further to study the phase behaviors of this binary system. The experimental VLE (PTx) data were measured from 205 to 248 K for six concentrations. Single gaseous phase PTVx properties for four concentrations were measured using a constant volume cell: densities of 100, 102, 146, and 224 kg m(-3) at temperature ranges of 333-385 K. The experimental data, combined with the previously reported data, were analyzed with a cubic equation of state, where non-conventional mixing rules were required in order to fit the experimental VLE data satisfactorily. The VLE phase behavior is discussed with ammonia mixtures with other compounds. There are strong evidences that some of HFCs and HCFCs form complexes (or even react) with ammonia. The complex formation and VLE behavior are elusively related to the number of fluorine atoms and the molecular structure. For ammonia/R-125, it is estimated the enthalpy of complex formation is about -10 kJ mol(-1), which is in concordance with those for hydrogen-bonding and/or charge-transfer complexes reported in the literature.