The binary Flory-Huggins interaction parameters for the ternary membrane-forming system nylon, formic acid and water have been obtained from literature data, swelling values and melting point depression. Nylon 4,6 nylon 6 and a copolymer of nylon 4,6 and 6 were examined. The isothermal crystallization boundaries were determined experimentally and the binodal miscibility gap was calculated for these ternary systems. It was found that the crystalline state is the thermodynamically favourable state for each system. Experimental data and calculated phase diagrams are discussed in relation to membrane formation. Although the thermodynamic properties of the system dictate the phase separation that can take place, the kinetic parameters are of equal significance with respect to membrane formation.