Molecular composites were prepared by the solution blending of poly(ethylene oxide) (PEO) of three different molecular weights with low concentrations of poly(p-phenylene terephthalamide) (PPTA) anion serving as a rigid-rod reinforcement. For all of the composites, the tensile strength, stiffness, and fracture energy were enhanced relative to values for the three PEO homopolymers. Composites made with PEO of high molecular weight (similar to 10(6) or higher) exhibited good extensibility and improved resistance to stress relaxation and attained maximum values of strength and toughness at a PPTA-anion content of about 6 wt %. The enhancement in mechanical properties is primarily attributed to good miscibility between the components arising from the presence of intermolecular, ion-dipole interactions between the ionic units of the PPTA anion and the dipole units of the PEO. Some consideration is also given to possible effects of a nucleating agent and an annealing treatment, both of which lead to additional increases in the tensile strength of the composites.