Abstract: A rubber-steel cord adhesion promoter represented by the general formula ##STR1## wherein Y.sub.1, Y.sub.2 and Y.sub.3 are identical or different and each represents a resin acid radical, a naphthenic acid radical or an aliphatic monocarboxylic acid radical having 7 to 24 carbon atoms provided that Y.sub.1, Y.sub.2 and Y.sub.3 are not simultaneously aliphatic monocarboxylic acid radicals having 7 to 15, particularly 7 to 11, carbon atoms. The promoter aids in imparting a high rubber-to-steel cord adhesive strength which is retained for an extended period of time even at high temperature and humidity.