The kinetics of O3 formation after an electron pulse have been studied in O2, at pressures from 7 to 400 kN m–2, by optical absorption measurements in the range 250–330 nm. The absorption spectra at different times and the time-dependence of the absorption at 250, 270 and 310 nm are interpreted in terms of three, kinetically distinguishable, consecutive reactions: O + O2+ O2 [graphic omitted] Oα3+ O2, α3+ O2 [graphic omitted] Oβ3+ O2, Oβ3+ O2 [graphic omitted] O3+ O2. The intermediates, Oα3 and Oβ3, which are assumed to be vibrationally excited O3, have broad absorption with maxima at approximately 310 and 285 nm respectively. The kinetic analysis leads to a value of k1= 5.4 ± 0.3 × 10–34 cm6 molecule–2 s–1 and also gives values for the rate constants of the reactions which deactivate excited O3. In mixtures of O2 with various other gases, values have been obtained for the rate constants of the reactions O + O2+ M[graphic omitted]Oα3+ M, k1a= 2.7 ± 0.3, 13.0 ± 1.5, 13.3 ± 1.2, 31 ± 3 × 10–34 cm6 molecule–2 s–1 for M = Ar, N2O, CO2, SF6 respectively. CO2 and SF6 are efficient deactivators of excited O3; values are reported for the rate constants for deactivation of Oβ3 by these molecules.