Poly(vinyl chloride) (PVC), poly(tetramethylene glycol) (polytetrahydrofuran, PTHF), and their blend were studied at 120 degrees C by inverse gas chromatography (IGC). The retention behavior of the homopolymers and their blend (0.5 PTHF volume fraction) was characterized for 38 probes. The IGC-derived apparent polymer-polymer interaction parameters chi(23)＇, ranging from -0.075 to 1.094, depended on the nature of the probe. The specific retention volumes V-g of the blend were found to be very close to the weight-average V-g values of PVC and PTHF suggesting a phase separation. The phase separation in the blend was confirmed by light scattering measurements. On the basis of the equation V-g,V-blend = W2Vg,2 + W3Vg,3, an approximate expression, chi(23)＇ (chi(12) -chi(13))(2)/2, was Obtained for phase-separated systems and was verified by IGC results for the PVC-PTHF blend. The apparent probe dependence of chi(23)＇ for this system was well explained.