The mechanism of adhesive bonding of rubber to metal using an interlayer of bonding agent (adhesive) is discussed with respect to various physical and chemical events such as adsorption at the metal surface, chemical crosslinking within the adhesive, interdiffusion, and formation of interpenetrating networks at the rubber-adhesive interface. An investigation on the peel strength of a natural rubber (NR)adhesive-metal joint, made by vulcanization bonding using nitrile-phenolic adhesive containing various concentrations of toluene diisocyanate-nitrosophenol (TDI-NOP) adduct, is presented. A single-coat adhesive, consisting of a p-cresol phenol formaldehyde resin, nitrile rubber (NBR), and vulcanizing agents in methyl ethyl ketone solvent, was selected for the study. Considerable improvement in the peel strength was obtained by the incorporation of TDI-NOP adduct into the nitrile-phenolic adhesive. The peel strength increases as the concentration of TDI-NOP adduct in the adhesive composition increases, then levels off with a transition from interfacial failure to cohesive tearing of rubber. The peel strength improvement is believed to be attributed to the interfacial reactions between the bonding agent and natural rubber, when TDI-NOP adduct is incorporated.