A biodegradable heterophase polymeric composition having good resistance to ageing and to low humidity conditions, the composition comprising a thermoplastic starch and a thermoplastic polymer incompatible with starch, wherein the starch is in the form of particles having an average numerical dimension of less than 1 μm and is in a dispersed phase and the thermoplastic polymer is in a continuous phase, wherein the thermoplastic polymer is a copolyester selected from the group consisting of aliphatic-aromatic copolyesters comprising from 30 to 70% by moles of aliphatic structure and wherein the aromatic structure derives from terephthalic acid and/or isophthalic acid, polyester-amides deriving from 30-70% by weight from an aliphatic amide, polyester-ethers, polyester- ether-amides, polyesterurethanes, and polyester-ureas,
- wherein the composition is an extruded composition having a water content of from 1 to 5% by weight measured at the exit of the extruder, prior to any conditioning, and wherein the decrease of the molecular weight of the thermoplastic copolyester is negligible.