Sulfonation of poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene), PEEK, improves its miscibility with poly(epsilon-caprolactam), Nylon-6 (N6). This article describes the thermal transitions and the specific interactions that occur for blends of the free acid derivative (H-SPEEK) and the lithium (Li-SPEEK) and zinc salts (Zn-SPEEK) of sulfonated PEEK (19.2 mol % sulfonation) with N6. The interactions responsible for miscibility were characterized by Fourier transform infrared (FTIR) spectroscopy. For blends of H-SPEEK and N6, miscibility is due to hydrogen bonding between the sulfonic acid and the amide group. For blends of N6 with the salts of SPEEK the specific interaction involves an ion-dipole complex of Li+ with the amide carbonyl or Zn2+ with the amide nitrogen. The relative strengths of the intermolecular interactions for the three types of blends increased as the cation was varied in the order : H+ < Li+ < Zn2+, and the T(g)s of the mixtures increased in the same order.