A masterbatch composition comprising an aromatic polyester resin, a polyolefin and a polyfunctional compound selected from the dianhydrides of tetracarboxylic aromatic and/or aliphatic acids and the polyepoxidic compounds, wherein the polyester resin and the polyfunctional compounds are omogeneously dispersed as solids in a matrix formed by the polyolefin, as domains of average particle size less than 800μ. The masterbatch is prepared by milling polyester resins and polyolefin granules to obtain powders having average particle size of less than 200μ, and the powders are therafter homogeneously mixed.