Process for producing an impact-modified thermoplastic moulding compound comprising A) 30% to 94% by weight of a polymer or polymer mixture consisting of at least one representative selected from aromatic polycarbonate and aromatic polyestercarbonate and optionally additionally comprising aromatic polyester, B) 5% to 65% by weight of a rubber-based graft polymer B.1, optionally in a mixture with rubber-free vinyl (co)polymer B.2, D) 0% to 30% by weight of one or more polymer additives, wherein the components are mixed with one another and melted and interdispersed at a temperature of 200 to 350°C in a compounding unit, characterized in that, in the process, component C added to the composition is an ester of a sulfonic acid in a concentration of 0.001% to 1% by weight and components A, B, C and D and any further components of the composition are mixed in a single step or else, alternatively, component C is first premixed in a first step with the entirety or a portion of component A at room temperature or at an elevated temperature, wherein component A used is aromatic polycarbonate and/or aromatic polyestercarbonate containing Fries structures of at least one of the formulae to in which the phenyl rings may each independently be mono- or disubstituted by C1-C8-alkyl, or halogen, and where A is a single bond, C 1 to C 5 -alkylene, C 2 to C 5 -alkylidene, C 5 to C 6 -cycloalkylidene, -O-, -SO-, -CO-, -S-, -SO 2 -, C 6 to C 12 -arylene, onto which further aromatic rings optionally containing heteroatoms may be fused, or a radical of formula or where R 5 and R 6 can be chosen individually for each X 1 and are independently hydrogen or C 1 to C 6 -alkyl, X 1 is carbon and m represents an integer from 4 to 7, with the proviso that, on at least one X 1 atom, R 5 and R 6 are both alkyl, wherein component A contains structural units to in an amount that adds up to 50 to 10 000 mg/kg, based on the sum total of the proportions by weight of the polycarbonates and polyestercarbonates present in component A and wherein component B has a content of alkali metals other than lithium that adds up to < 10 mg/kg. A) 30% to 94% by weight of a polymer or polymer mixture consisting of at least one representative selected from aromatic polycarbonate and aromatic polyestercarbonate and optionally additionally comprising aromatic polyester, B) 5% to 65% by weight of a rubber-based graft polymer B.1, optionally in a mixture with rubber-free vinyl (co)polymer B.2, D) 0% to 30% by weight of one or more polymer additives, wherein the components are mixed with one another and melted and interdispersed at a temperature of 200 to 350°C in a compounding unit, characterized in that, in the process, component C added to the composition is an ester of a sulfonic acid in a concentration of 0.001% to 1% by weight and components A, B, C and D and any further components of the composition are mixed in a single step or else, alternatively, component C is first premixed in a first step with the entirety or a portion of component A at room temperature or at an elevated temperature, wherein component A used is aromatic polycarbonate and/or aromatic polyestercarbonate containing Fries structures of at least one of the formulae to in which the phenyl rings may each independently be mono- or disubstituted by C1-C8-alkyl, or halogen, and where A is a single bond, C 1 to C 5 -alkylene, C 2 to C 5 -alkylidene, C 5 to C 6 -cycloalkylidene, -O-, -SO-, -CO-, -S-, -SO 2 -, C 6 to C 12 -arylene, onto which further aromatic rings optionally containing heteroatoms may be fused, or a radical of formula or where R 5 and R 6 can be chosen individually for each X 1 and are independently hydrogen or C 1 to C 6 -alkyl, X 1 is carbon and m represents an integer from 4 to 7, with the proviso that, on at least one X 1 atom, R 5 and R 6 are both alkyl, wherein component A contains structural units to in an amount that adds up to 50 to 10 000 mg/kg, based on the sum total of the proportions by weight of the polycarbonates and polyestercarbonates present in component A and wherein component B has a content of alkali metals other than lithium that adds up to < 10 mg/kg.