An aqueous copolyester dispersion suitable for the subbing of polyester film without use of organic solvents wherein the copolyester contains recurring ester groups derived from ethylene glycol and an acid mixture essentially consisting of 20 to 60 mole % of isophthalic acid, 6 to 10 mole % of sulphoisophthalic acid whose sulfo group is in salt from, 0.05 to 1 mole % of an aromatic polycarboxylic acid compound having at least three carboxylic acid groups, which compound is not capable of forming intramolecularly an anhydride and includes at least two carboxylated aromatic nuclei, and the balance of terephthalic acid; the copolyester has a glass transition temperature of at least 50.degree. C. and an intrinsic viscosity of from 0.15 to 0.45 dl/g when measured at 25.degree. C. in a mixture of phenol/o-dichlorobenzene (60/40 by volume).