The microscopic structure of amorphous polymers was studied by using different molecular probes in finite dilution inverse gas chromatography (FDIGC) experiments. This molecular probe technique allows one to study nonrandom partitioning of solvent molecules and spatial correlation of polymer chain segments in a bulk. A novel approach was developed to determine cluster integrals as defined in the Kirkwood−Buff theory of mixtures. Solvent self-cluster integrals, a combination of cluster integrals expressing preferential solvation, and the values of the mean cluster size were determined for different polymer systems as a function of the volume fraction of the probe in the bulk of the polymers. Polymers with different polarities [a substituted poly(thionylphosphazene), a saturated polyester poly(diethylene glycol−succinic acid), and an essentially apolar poly(dimethylsiloxane)] were used as model materials. The microstructure and the solvent−polymer interaction are discussed in detail for each system. The results obtained allowed us to gain novel insight into the polymeric microstructure and short-range order, thus giving credibility to our approach to study microstructure by cluster integrals from inverse gas chromatography (IGC) data.