A polysiloxane-polycarbonate block cocondensate derived from at least one hydroxyaryl-terminated siloxane of the formula (siloxane component) in which R 1 is H, Cl, Br or C 1 to C 4 -alkyl, R 2 and R 3 are the same or different and each independently from one another selected from aryl, C 1 to C 10 -alkyl and C 1 to C 10 -alkylaryl, X is a single bond, -CO-, -O-, C 1 - to C 6 -alkylene, C 2 to C 5 -alkylidene, C 5 to C 12 -cycloalkylidene or C 6 to C 12 -arylene which may optionally be fused to further aromatic rings containing heteroatoms, n is a number from 1 to 500, m is a number from 1 to 10, and p is 0 or 1; and at least one polycarbonate wherein the polysiloxane-polycarbonate block cocondensate contains at least one of the following structures to: in which the phenyl rings are unsubstituted or independently mono- or disubstituted by C 1 to C 8 -alkyl and/or halogen, X is a single bond, C 1 to C 6 -alkylene, C 2 to C 5 -alkylidene or C 5 to C 6 -cycloalkylidene, the linkages indicated by --- in the structural units to are each part of a carboxylate group; wherein the amount of the structural units to totals 60 to 1500 ppm as determined after hydrolysis by HPLC, based on the polysiloxane-polycarbonate block cocondensate and which has a sodium content within the range of from 0.1 ppm to 1000 ppm as determined by atomic absorption spectroscopy with flame atomization. at least one hydroxyaryl-terminated siloxane of the formula (siloxane component) in which R 1 is H, Cl, Br or C 1 to C 4 -alkyl, R 2 and R 3 are the same or different and each independently from one another selected from aryl, C 1 to C 10 -alkyl and C 1 to C 10 -alkylaryl, X is a single bond, -CO-, -O-, C 1 - to C 6 -alkylene, C 2 to C 5 -alkylidene, C 5 to C 12 -cycloalkylidene or C 6 to C 12 -arylene which may optionally be fused to further aromatic rings containing heteroatoms, n is a number from 1 to 500, m is a number from 1 to 10, and p is 0 or 1; in which R 1 is H, Cl, Br or C 1 to C 4 -alkyl, R 2 and R 3 are the same or different and each independently from one another selected from aryl, C 1 to C 10 -alkyl and C 1 to C 10 -alkylaryl, X is a single bond, -CO-, -O-, C 1 - to C 6 -alkylene, C 2 to C 5 -alkylidene, C 5 to C 12 -cycloalkylidene or C 6 to C 12 -arylene which may optionally be fused to further aromatic rings containing heteroatoms, n is a number from 1 to 500, m is a number from 1 to 10, and p is 0 or 1; and at least one polycarbonate wherein the polysiloxane-polycarbonate block cocondensate contains at least one of the following structures to: in which the phenyl rings are unsubstituted or independently mono- or disubstituted by C 1 to C 8 -alkyl and/or halogen, X is a single bond, C 1 to C 6 -alkylene, C 2 to C 5 -alkylidene or C 5 to C 6 -cycloalkylidene, the linkages indicated by --- in the structural units to are each part of a carboxylate group; wherein the amount of the structural units to totals 60 to 1500 ppm as determined after hydrolysis by HPLC, based on the polysiloxane-polycarbonate block cocondensate and which has a sodium content within the range of from 0.1 ppm to 1000 ppm as determined by atomic absorption spectroscopy with flame atomization. wherein the polysiloxane-polycarbonate block cocondensate contains at least one of the following structures to: in which the phenyl rings are unsubstituted or independently mono- or disubstituted by C 1 to C 8 -alkyl and/or halogen, X is a single bond, C 1 to C 6 -alkylene, C 2 to C 5 -alkylidene or C 5 to C 6 -cycloalkylidene, the linkages indicated by --- in the structural units to are each part of a carboxylate group; the phenyl rings are unsubstituted or independently mono- or disubstituted by C 1 to C 8 -alkyl and/or halogen, X is a single bond, C 1 to C 6 -alkylene, C 2 to C 5 -alkylidene or C 5 to C 6 -cycloalkylidene, the linkages indicated by --- in the structural units to are each part of a carboxylate group; wherein the amount of the structural units to totals 60 to 1500 ppm as determined after hydrolysis by HPLC, based on the polysiloxane-polycarbonate block cocondensate and which has a sodium content within the range of from 0.1 ppm to 1000 ppm as determined by atomic absorption spectroscopy with flame atomization.