An experimental and theoretical investigation of the phase behavior of blends of aliphatic polyesters, possessing different methylene content and chlorinated polypropylenes (PP-Cl), containing 26, 32 and 65 wt% chlorine, respectively, has been performed. A window of miscibility was found only for the PP-Cl65; however, this was sufficient to apply a binary interaction model of phase behavior in polymers and allow for an estimation of the segmental interaction parameters that define the overall polymer pair interaction. Complete immiscibility was found in all blends of polyesters containing PP-Cl26 and PP-Cl32; a result consistent with model calculations. A number of simplifications have been made concerning the structures of polymers and assumptions have been made to apply the theoretical model. The segmental interaction parameters obtained have been extended to estimate the thermodynamic interaction in blends of polyolefins and chlorinated polyolefins and then used to calculate the relevant interfacial thicknesses. These results have been compared with information presented in the literature.