The present invention refers to a paper surface sizing composition comprising a copolymer having: a) structural units derived from ethylenically unsaturated hydrocarbons; structural units derived from monomers selected from esters of ethylenically unsaturated mono-carboxylic acids, half-esters of ethylenically unsaturated dicarboxylic acids, allylethers and vinylethers and mixtures thereof; optionally c) structural units derived from ethylenically unsaturated monomers selected from monocarboxylic acids as well as salts and amides thereof, dicarboxylic acids as well as salts, amides and half-amides thereof, and cyclic anhydrides and imides of dicarboxylic acids and mixtures thereof; and optionally d) structural units derived from alkyl or amino-substituted alkyl acrylates or methacrylates with the proviso, that if no structural units c) are present, the structural units b) are derived from monomers selected from half esters of ethylenically unsaturated dicarboxylic acids.