FTIR spectroscopy was applied to the study of the high-pressure reactivity of solid butadiene. The chemical transformation from the ordered phase I was observed to occur only above 270 K. The existence of a threshold temperature for the reaction reveals the central role of the lattice phonons in the activation of the transformation. Below 4.0 GPa only dimerization to 4-vinylcyclohexene occurs, while above this pressure an increasing amount of polymer forms with rising pressure. Room temperature kinetic studies have been performed at different pressures, from 2.1 up to 6.6 GPa, and the sign of the activation volume for the dimerization has been obtained. The dimerization reaction is found to follow a first-order mechanism. A reaction pathway for this process is proposed where the internal rearrangement of a diradical intermediate specie is identified as the rate limiting step. An acceleration of the dimerization process is observed above 4.0 GPa and is ascribed to the simultaneous polymer formation. This effect causes the laser assisted reaction, where a large amount of polymer is produced at any pressure, to be not as selective on polymerization as it is in the liquid phase, since also the dimerization rate is enhanced. (C) 2003 American Institute of Physics.