The compatibility behaviour of melt-mixed blends of a polyester-type polyurethane (PU) with the Zn2+ ionomer of poly(ethylene-co-methacrylic acid-co-isobutylacrylate) terpolymer (Ion., Zn2+) was investigated in the complete composition range. The techniques applied were tensile testing, dynamic mechanical analysis (d.m.a.), differential scanning calorimetry (d.s.c.), Fourier transform infrared (FTi.r.) spectroscopy and optical microscopy. Tensile properties were typical of a compatible system and d.m.a. results indicated a miscible blend. FTi.r. results failed to reveal the nature of the specific forces involved in bringing about compatibilization. Analysis of the thermal data obtained allowed the determination of a polymer-polymer interaction parameter which had a near zero value at the elevated temperatures where it was determined.