The ambient temperature photoinduced polymerization of compressed (P < 1 GPa) fluid ethylene was characterized by transient infrared absorption spectroscopy with a resolution of few nanoseconds, 3 orders of magnitude higher than previously reported. The reaction has been studied under both one- and two-photon excitation evidencing in the latter case its occurrence only in the presence of different transition metal oxides. Their photocatalytic activity is ascribed to the stabilization of the excited biradicals through electron density exchange between the d orbitals of the metal and the π antibonding orbitals of ethylene which lengthens the lifetime of the biradicals. In both one- and two-photon activation cases the polymerization is characterized by an initial step distinguished by a molecularity of 0.15 ± 0.02 identified as the activation step of the reaction lasting, in the one-photon excitation case, a few hundreds of nanoseconds. Using pulsed excitation the reaction evolves toward a free radical polymerization only under one-photon excitation whereas the critical concentration of radicals required to propagate the reaction is never achieved in the two-photon excitation case. Comparison with continuous wave excitation unambiguously identifies in the average power released to the sample the key factor to drive quantitatively and qualitatively the polymerization.