Solution blends of a modified polyacrylonitrile (PAN) with polyurethane (PU) ionomers were prepared inN,N-dimethylformamide. PU anionomer (PUAn20) blends showed phase separation of up to several micrometers, whereas almost homogeneous morphology was obtained with PU ca-tionomer blends. In PU cationomer blends, the tan δ peaks of the soft segments of PU and PAN moved toward lower and higher temperature, respectively. Notably, the tensile strength of PU cationomer blend at 30/70 (PU/PAN by weight) increased to over 2.5 times that of PAN; for this blend the concentration of PU cation approximately matched that of the sulfonate anion of the modified PAN. The synergistic effect was interpreted in terms of ionic interactions between the main chains of the polymers.