Molecular composites were prepared from several types of ionically modified, poly(p-phenylene terephthalamide) (PPTA) dispersed in a poly(4-vinylpyridine) matrix. Optical clarity tests indicated that the component polymers of the composite were miscible, at least at low concentrations of the rodlike reinforcement, In composites containing ionic PPTA, where ionic sulfonate groups were attached as side groups either to PPTA chains or to PPTA anion chains, the glass-transition temperature (T-g) was increased by 10 degreesC or more, at 5 wt% reinforcement. At concentrations of 10-15 wt % of the ionic polymer, T-g values leveled off or decreased slightly. This suggested that some aggregation of the rigid-rod molecules occurred. In composites containing ionic PPTA, where the ionic sulfonate groups were directly attached to the phenylene rings of PPTA chains, not only was T-g shifted significantly to higher temperatures, but the rubbery plateau modulus retained high values up to temperatures of 250 degreesC or above. Observed effects were considered to be the result of strong ionic interactions between the ionic reinforcement polymer and the polar matrix polymer. The possible effects of the counterion on T-g and the storage modulus are discussed.