The kinetics of the reaction O 2 ( 1 ∆ g ) + O 3k2 → 2O 2 + O have been investigated in the temperature range 195 to 439 K by using the kinetic photoionization technique to follow [O 2 ( 1 ∆ g )]. In Arrhenius form, the rate constant, k2 ,'is given by k2 = 4.0 ± 1.5 x 10 8 exp (13000 /RT) 1 mol -1 s -1 (joule units) ( = 4.0 ± 1.5 x 10 8 exp (3100/RT) 1 mol -1 s -1 (calorie units)). At room temperature (292 K) k 2 = 2.1 ± 0.3 x 10 6 1 mol -1 s -1 . The activation energy of 13 ± 1.6 kJ mol -1 suggests that there is virtually no barrier to the reaction other than that provided by its endothermicity (12.1 kJ mol -1 ). The results are used to derive, from previously published data, a value of the rate constant for the reaction O + O 3k3 → 2O 2 of 4 ± 2 x 10 6 1 mol -1 s -1 at room temperature.