Strong bonds can from between natural rubber (NR) and nitrile rubber compounds when held together during vulcanization if the former is modified by the addition of liquid polybutene as a compatible extender. Successful bonding is also dependent on the vulcanization systems employed. It is suggested that the phenomenon be regarded as one of liquid-solid contact. A NR compound thus modified is sufficiently fluid to set, under moulding pressure, a high-viscosity nitrile rubber surface (notionally a solid) before vulcanization is well-established. As the NR compound possesses the lower surface tension (or energy), the system is thermodynamically favourable. Regarding the vulcanization systems, a fast-curing nitrile compound is required to maintain the notional solid role. Bond strengths decrease progressively as the NR vulcanization system is changed from conventional through sem,i-EV to EV; high strengths are apparently associated with the many polysulphidic crosslinks which form, for conventional systems, during early stages of vulcanization. It may require the relatively long lengths of polysulphidic crosslinks to traverse the bond interface. An alternative suggestion is that bond formation arises from the maturation reactions of polysulphidic ccrosslinks near the interface. Bonds were not affected by immersion in either sea water or ASTM No. 3 oil for thirty days at room temperature. The polybutene levels used did not reduce the NRs high tearing energy characteristics and reduced hardness and modulus by only 10;.