The tensile fracture properties of rigid-rigid blends made of sulfonated polystyrene (SPS) ionomer and polystyrene (PS) were studied. A positive deviation from the rule of mixtures in both tensile strength and tensile toughness was observed over the entire composition range, and an especially significant enhancement was observed at low ionomer composition (e.g., 10 wt %). This is attributed to a fine dispersion of the rigid ionomer phase in the PS matrix together with good interfacial adhesion between the phases.;The adhesion arises from entanglements due to athermal interactions between PS and the styrene units of the SPS ionomer chains. At a constant blend ratio, an increase in ion content of the ionomer component leads to an increase in both the tensile strength and toughness of the two-phase blends. The tensile properties of these blends appear to be determined largely by the resistance to fracture of the ionomer particles. Hence the addition of more fracture-resistant ionomers (e.g., ionomers of high ion content) to PS leads to blend materials of higher strength and toughness. Some enhancement of the mechanical properties may also be attributed to a stress concentration effect of the ionomer second-phase particles; which allow a greater volume of plastic energy to be absorbed in the blend materials. Changing the counterion of the ionomer particles from Na (monovalent) to Ca (divalent) results in a moderate increase in ultimate properties.