The kinetics of the reaction O + O2+ M → O3+ M (1) where M = O2, He, Ar, or CO2 have been determined at total pressures from 1 to 8 torr, the oxygen atoms being generated by pyrolysis of ozone at 1300°K. The rate constants observed in the temperature range 213–386°K when M = Ar and M = CO2 can be represented by kAr1=(1.7 ± 0.2)× 1013 exp (+1680 ± 100)/RT, and [graphic omitted] or alternatively by kAr1=(2.85 ± 0.3)× 1014(T/298)–3.0 ± 0.2, and [graphic omitted]. At 298°K the relative third-body efficiencies for equal molecular concentrations are O2: He : Ar : CO2=1.0 : 0.74 : 0.90 : 3.1. The values obtained for kAr1 are greater than those calculated from previous investigations of the kinetics of pyrolysis of ozone, even allowing for the accepted error in the equilibrium constant for reaction (1). Likewise the relative third-body efficiencies, with the exception of O2 : He, differ from those derived from earlier work on the pyrolysis and photolysis of ozone.