In order to develop a new polymeric material with a nice combination of high heat resistance, chemical resistance and flow, polysulfone (PSU)/polyamide blends have been investigated. The incompatibility of these polymers can be overcome by the addition of functionalized PSU, especially anhydride terminated PSU (PSU-PhA). Since PSU-PhA sometimes contains traces of low molecular weight compounds, the influence of such reactive impurities was investigated with phthalic anhydride (PhA) as model compound. Solvent extraction and electron microscopy results suggested that the amount of in situ created PSU-PA-copolymers is significantly reduced by the addition of PhA during the extrusion process resulting in a coarsening of the morphology.