Ionic liquids based on tetraalkylphosphonium salts combined with different anions (phosphinate versus trifluoromethylsulfonylimide) have been used as new compatibilizers of polymer blends. To highlight the effect of thermostable ionic liquids, a very low amount of ILs (1-10 wt%) have been introduced within a PP/PA blend (80/20) and the polymer blends have been processed in melt by twin-screw extrusion. Transmission electron microscopy (TEM) analysis has been used to investigate the influence of ILs on the different morphologies of these binary blends. In addition to having improved thermal stability, an excellent stiffness-toughness compromise has been obtained (+1400%). (C) 2014 Elsevier B.V. All rights reserved.