Aqueous polyurethane (PU) adhesives are nontoxic and nonflammable and do not pollute the air. However, they have low adhesive strengths compared to solvent-based PU adhesives because of a low affinity with rubber substrates. In this study, PU adhesives were synthesized from isophorone diisocyanate with dimethylol propionic acid as the ionic center in the main chain, triethylamine as the neutralization agent, and hydrazine as the chain extender. The polyol was modified with hydroxyl-terminated polybutadiene (HTPB) and hydroxyl-terminated acrylonitrile-butadiene copolymer (HTBN). The effect of the HTPB and HTBN content on the adhesive strength was investigated. The effect of the prepolymer molecular weight was also investigated. Increases in the HTPB and HTBN contents led to an increase in the adhesive strength because of the increase of chemical affinity between the adhesive and the substrate. The adhesive strength increased as the prepolymer molecular weight increased. This was due to an increase in the tensile strength and modulus. (C) 2005 Wiley Periodicals, Inc.