Polybutadiene rubber with 10% vinyl unsaturation has been crosslinked without the addition of any vulcanization chemicals. The crosslinks have been created through treatment at high temperature (240-250 degrees C) and elevated pressure (<293 MPa) in a plunger type mold. The treatment time needed to achieve a cross-linked material is of the order of a few minutes and the reaction rate is shown to be dependent upon the temperature as well as upon the pressure applied. As a result of the severe treatment, some degradation reactions may be expected but careful FTIR analysis shows no increased oxidation. There is some consumption of the vinyl unsaturation, to an amount comparable to the number of crosslinks formed. The presence of carbon black increases the measured crosslink density, as expected, but at higher pressures the amount of extra network material, because of the carbon black, is much larger than at lower pressures, suggesting that the carbon black plays a critical role in the network formation process. The stress relaxation properties in compression of the rubber material were investigated and found to be comparable to those of a peroxide vulcanizate.