Molecular composites have been prepared by mixing rigid rod molecules of PPTA anion (K-salt) and poly(propylene oxide) in a common solvent and compression molding the precipitated material. As the PPTA anion content of the composites is increased, an upward shift occurs in both the glass transition temperature and the decomposition temperature. The tensile strength, stiffness and resistance to stress relaxation of the composites are significantly increased, even at relatively low concentrations of the rigid rod reinforcement. The enhancement in mechanical properties results from a good dispersion of the rigid PPTA anion molecules and from the presence of ion-dipole interactions between the ionic groups of the PPTA anion and the dipolar units of the poly(propylene oxide) matrix. Additional increases in strength and rigidity may be achieved by changing the counterion from monovalent K+ to divalent Ca2+.