Ethylene is the simplest organic molecule containing a double bond and is the starting monomeric unit in the synthesis of polyethylene, one of the most largely produced polymers. Here we report a high pressure infrared study of ethylene at room temperature. A polymerization reaction is observed when the crystalline phase I is compressed above 3.0 GPa. The reaction kinetics was investigated at two different pressures, 3.6 and 5.4 GPa. The recovered product was identified in both cases as polyethylene, but while a conformationally disordered and branched low-density polymer is obtained at the highest pressure, a high-density crystalline polymer is obtained at 3.6 GPa. A reaction mechanism was proposed on the basis of the kinetic data and the structural information.