Molecular composites have been prepared by dispersing rigid-rod molecules of ionically-modified poly(p-phenylene terephthalamide) (PPTA anion) in a polar poly(4-vinylpyridine) (PVP) matrix. For concentrations up to 5 wt % of the rigid-rod reinforcement, the resulting composites are transparent and possess a single glass transition temperature that increases with concentration of the PPTA anion. The mechanical properties of the molecular composites are found to increase with concentration and to attain maximum values at about 5 wt % of the PPTA anion. The enhancement in properties, and the miscibility induced between the two component polymers, is attributed to the development of specific interactions between the ionic groups of the PPTA anion and the polar units of the PVP matrix. When such interactions are not present, as in composites reinforced with non-ionic PPTA, the samples are opaque and their properties are significantly reduced compared to those of the PPTA anion/PVP composites.