Compositions comprising A) from 40 to 80 parts by weight of aromatic polycarbonate or polyester carbonate, B) from 10 to 40 parts by weight of rubber-free vinyl copolymer, C) from 5 to 20 parts by weight of organically surface-modified chopped glass fibre, D) from 0 to 9 parts by weight of a graft polymer with an elastomeric graft base, E) from 0 to 20 parts by weight of additives, where the sum of the parts by weight of components A) to E) in the composition is standardized to 100, characterized in that (i) a size composed of an organic compound or a mixture of various organic compounds is used as surface-modification of the glass fibre at a concentration such that the carbon content of component C is from 0.2 to 2% by weight, (ii) the ratio of the integrated peak area of the FTIR spectrum in the wave number range from 2700 to 3000 cm−1 to the integrated peak area of the FTIR spectrum in the wave number range from 500 to 4000 cm−1, in each case measured on the dichloromethane-extracted content of this size of component C, is from 0.20 to 0.70, have improved toughness (impact resistance, notched impact resistance, performance on exposure to multiaxial impact and/or tensile strain at break) in comparison with the prior art and, because they have good melt flowability and increased tensile modulus of elasticity, are suitable for use in the production of mouldings.