Abstract The rate at which O 2( 1Δ g) is quenched by CO 2 is of great importance in interpreting air-glow studies of the atmospheres of both Mars and Venus. There has been a long-standing controversy in the literature about the rate coefficient for the quenching process. In this paper, further kinetic experiments using a flow system with direct detection of the O 2( 1Δ g → 3Σ g −) emission system are described. The upper limit obtained for the rate coefficient was 3.6 × 10 −19 cm 3 molecule −1 s −1. An explanation is advanced for the earlier discrepancies; this explanation is concerned with the use of energy-sensitive, but not specific, detection methods for the excited species.