Mechanical properties, notably the ductility of rutile-filled low-density polyethylene (LDPE) and chlorinated polyethylene (CPE), were studied to determine the influence of filler dispersion and of matrix-filler interactions. Potential interactions between polymer and filler were obtained from inverse gas chromatography, in the form of acid/base interaction parameters. The strength of filler aggregates was obtained from flow properties of the dry powders. Dispersion quality in molded specimens was identified from X-ray contact micrographs. The ductility of filled, nonpolar LDPE was found to be independent of acid/base considerations, but strong correlations have been drawn between the strength of filler aggregates and the quality of dispersion attained by controlled mixing methods. Inherent filler agglomeration thus also affects the mechanical properties of the nonpolar matrix. Coated, nonagglomerated fillers dispersed well, leading to ductilities at low filler load that exceeded that of the unfilled host polymer. In contrast, acid/base interactions were found to exert dominant influence on rutile dispertion in CPE. Similarly, the mechanical properties of this polymer varied with the acid/base forces acting at matrix/filler contacts.