Non-random partitioning of molecular probes in a polymeric mixture was examined with Kirkwood-Buff-Zimm (KBZ) cluster integrals. Equations were derived to give relationships of thermodynamic quantities that can be obtained by using inverse gas chromatography (IGC). The mer-mer correlation structure factor S*(AB) (0) in a binary mixture of polymers A and B in terms of the spatial distribution of a probe, S, around a molecule A and a molecule B is discussed. The derivation allows a direct assessment of the correlation structure factor obtained either from scattering experiments or from IGC measurements. The common polymer-polymer interaction parameter from the two types of experiments is discussed with respect to criticism concerning IGC. The detailed molecular description of the probe behavior in a polymeric mixture at infinite dilution of the probe given in this work can be used in IGC studies of the microstructure of polymer mixtures or other mixtures of liquids. In order to illustrate our approach, the structure factor and the interaction parameter of the blends of polystyrene (A) and poly(butyl methacrylate) (B) were evaluated using IGC results of DiPaola-Baranyi and Degre.