AbstractThe compatibility behavior of polyetherether ketone (PEEK) with poly(ether sulfone) (PES) has been reexamined using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and extrudate swell measurements. The blends were prepared by both melt‐blending and solution‐blending techniques. The phase behavior of blends is strongly affected by the blending technique used. Blends prepared by solution‐blending are compatible in the entire composition range on the basis of the single composition‐dependent glass transitions and exhibit lower critical solution temperature (LCST) behavior. LCST was near 340 °C around which the crystalline melting point of PEEK exists. Near LCST melting‐induced movement of molecular chains disturbs the initial homogeneous state of the solution blends and leads to a phase‐separated state that is thermodynamically more stable in the absence of strong specific interactions between the homopolymers. Contrary to the solution‐blended samples, melt‐blended samples were in the phase‐separated state even at a lower processing temperature of 300 °C. Two glass transitions corresponding to a PEEK‐rich and a PES‐rich phase were found for all compositions. From the measured glass transition of phase‐separated blends, weight fractions of PES and PEEK dissolved in each phase were determined using the Fox equation. Compatibility is greater in the PEEK‐rich compositions than in the PES‐rich compositions. PEEK dissolves more in PES‐rich phases than does PES in the PEEK‐rich phase. Variation of the specific heat increment (ΔCp) at the glass transition with composition also supports these inferences. Solution‐blended samples, quenched from 380 °C, also indicated similar behavior but were slightly more compatible. The aforementioned results are consistent with those inferred from SEM studies and extrudate swell measurements that show a greater compatibility in PEEK‐rich compositions than in PES‐rich compositions. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1407–1424, 2002