It has been known that blends of poly(epsilon-caprolactone) (PCL) and poly(styrene-co-acrylonitrile) (SAN) show a miscibility window in a relatively narrow range of copolymer composition in SAN. In order to study the interactions in this miscibility window in detail,FTIR and calorimetric measurements were carried out by using low molecular weight analogues corresponding to monomers of each polymer, because respective homopolymer pairs are not miscible. The FTIR study showed shifts of spectral peaks that is thought as a proof of presence of specific interactions. The acid-base self-interaction energies (E(ii)) and association energies for respective pairs (E(ij)) were also estimated by the detailed study of interactions by calorimetry. If the FTIR results were combined with the calorimetric ones, the acid-base spectral shifts of peaks, Delta v(ab) (C=O stretch, C=N stretch, and benzene ring out-of-plane C-H bending), indicated a linear relation with E(ij) values that fits well with Drago’s treatment based on quantum mechanics. The data of two independent experiments justified the presence of specific interactions. Finally the heats of mixing of homopolymer-copolymer pairs were calculated by using the self-interaction and association energies and compared with the behavior of miscibility window.