A thermoplastic molding composition having improved impact strength and a process for its preparation are disclosed. The structure of the resinous component of the composition contains carbonate structural units, polyester structural units and lactone-siloxane structural units conforming to ##STR1## wherein m is 1 to 90, n is 5 to 300, R is an alkyl or a phenyl radical, p is 2 to 12 and q is 2 to 20. The preparation of the resinous component entails reactive blending in the presence of a transesterification catalyst of polycarbonate, (co)polyester and a lactone siloxane copolymer.