Ethylene is polymerized with thermal and with UV-photochemical initiation at temperatures between 130°C and 255°C up to pressures of 2500 bar. The conversions are between two and five percent. The degrees of polymerization as obtained from viscosity experiments are dependent on reaction temperature and pressure, but are insensitive to the kind of initiation, thermal or photochemical. These observations are explained by an equation which relates the degree of polymerization to kinetic parameters of the system which have partly been determined by independent experiments. From this equation, the activation energy and the activation volume of the transfer constant with the monomer, cm, are determined to be Ea(cm) = 43,9 ± 4 kJ · mol-1 and ΔV≠ (cm)=20,1 ± 3 cm3 · mol-1, respectively.