Mixtures of nylon 6 and lightly sulfonated polystyrene ionomers (M-SPS, where M = Li, Mg, Zn or Mn) exhibit lower critical solution temperature (LCST) phase behaviour. Miscibility was a consequence of specific complex formation between the metal sulfonate and amide groups. The interactions were strongest when the cation was Zn2+ or a transition metal such as Mn2+. The LCST increased as the sulfonation level increased and as the cation was changed in the order of Mg2+ < Li+ < Zn2+ < Mn2+. Despite large differences in the molecular weights of the two polymers, the liquid-liquid phase diagrams were symmetric about the 50/50 composition. The glass transition temperature of miscible blends showed a positive synergism due to the physical network resulting from intermolecular complex formation. Although both polymers were hygroscopic, the blends exhibited less water absorption than either of the two components alone, which was a consequence of the complex removing sites for hydrogen bonding of water. The amount of water absorption decreased when the cation was varied in the order of Mg2+ > Li+ > Zn2+.