A process for sizing staple fiber yarns by applying an aqueous sizing liquor which contains an alkaline earth metal salt of a copolymer containing from 90 to 30% by weight of acrylic acid and/or methacrylic acid and from 10 to 70% by weight of acrylonitrile and/or methacrylonitrile, which copolymer has a viscosity of from 50 to 1,000 cp in 15% strength aqueous solution at 85 DEG C., and drying the treated yarns. The sized films obtained are moisture-insensitive and have good heat stability.