High Pressure Vulcanisation (HPV) is referred to as crosslinking of rubber materials merely by treatment at high temperature and elevated pressure, thus without any vulcanisation agents. The reaction mechanisms in crosslinking of polybutadiene by HPV have been penetrated and compared with the crosslinking at different pressures when peroxide is present. The effect of pressure on the reactions has been investigated in terms of molecular mobility, and the heterogeneity of the network has been examined. It was found that the two vulcanisation processes are very similar and that they both proceed mainly by addition to vinyl unsaturations. They are, however, different with regard to the initiation step, and this is shown to lead to a more homogeneous network in the purely HPV-crosslinked polybutadienes. The crosslinking efficiency of peroxide vulcanisation at 170 degrees C was calculated to vary between 8 and 12 as the pressure increased from 26 to 293 MPa which is the pressure range that has been investigated.