A series of pure ethylene polymerizations performed to maximum pressures of 2,5 kbar and temperatures up to 250°C is discussed. The rate data which are obtained via quantitative infrared and near infrared spectroscopy closely fit to an overall polymerization rate law of the order 2,5 ± 0,2 in the ethylene concentration. The overall activation energy is determined to be 136,3 ± 6 kJ/mol. For the initiation reaction an activation energy of 209,8 ± 12 kJ/mol is derived. This fairly large value establishes that in dense high purity ethylene a thermally initiated (spontaneous) reaction to high molecular weight material occurs.