The invention comprises an adhesion promoter for a printing ink and a printing ink comprising the adhesion promoter. The adhesion promoter comprises the product of (a) mixing together a solution of a polymer or synthetic resin with an organophosphorus compound, and then (b) mixing the mixture formed in (a) with a titanium compound selected from a titanium halide, alkoxide, halo-alkoxide or a condensed titanium alkoxide or a mixture of more than one of these titanium compounds.