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 reacting together (a) a solution of a polymer or synthetic resin (b) a sulphonic acid or a derivative of a sulphonic acid, and (c) a metal compound selected from one or more of the group consisting of a metal halide, metal alkoxide, metal halo-alkoxide or a condensed metal alkoxide, wherein the metal is selected from titanium or zirconium.