Inverse gas chromatography (IGC) was used to analyze the secondary transition temperatures and the miscibility of binary mixtures of poly (ether imide) (Ultem (TM)) and a copolyester of bisphenol-A with terephthalic and isophthalic acids (50/50) (Ardel (TM)) in three compositions (25/50, 50/50 and 75/25). Retention diagrams of the mixtures of Ultem (TM) and Ardel (TM) for n-nonane, n-decane, n-butyl acetate and isoamyl acetate were obtained at temperatures between 60 and 285 A degrees C. Second-order transition temperatures of the mixtures were determined according to the slope change in retention diagrams of the solvents. The glass transition temperatures of the mixtures suggested the miscibility of the polymers. Polymer-polymer interaction parameters of binary mixtures of the polymers were determined at temperatures between 260 and 285 A degrees C by Flory-Huggins theory. The polymer-polymer interaction parameters were dependent on the solvent used. The small values of polymer-polymer interaction parameters close to zero suggest some weak interactions between the polymers in the mixture. It was concluded that it was possible to obtain more meaningful information related to the interactions of polymers in a mixture from IGC measurements, if binary polymer-solvent interaction parameters of the used solvent probes were around 0.5.