Method of insulating an electrical conductor

A method of insulating an electrical conductor which comprises applying to said conductor a coating of a polyesterimide resin which can be hardened through its free hydroxyl groups and which may also contain amide groups, from a melt of the resin at a temperature above 100.degree. C. using a heatable application means, wherein the resin used is solvent-free, and has been prepared by the esterification or ester-exchange of starting materials for the polyesterimide in the presence of an excess of one or more short-chain diols, and subsequent removal of the diol or diols in such manner that the condensation is only effected to an extent such that the Durrans softening point of the unhardened resin is not greater than 150.degree. C., and that the viscosity of the molten resin at 180.degree. C. is not greater than 5000 mPa s (cP), the equivalent ratio of hydroxyl groups to ester-forming carboxyl groups for the starting materials used to form said resin being in the range of 3:1 to 8:1.