The deactivation rate constants of O 2 ( 1 Δ g ) by H 2 and D 2 are respectively measured to be 3.91±0.12×10 −18 cm 3 s −1 and 1.1±0.3×10 −19 cm 3 s −1 , which agree well with the quantum mechanical calculation by Kustrev. Their deactivation mechanisms by H 2 and D 2 are analyzed according to the E–V energy transfer model developed by Schmidt, indicating that the main deactivation channel for H 2 is O2(1Δg,v=0→3Σg-,v″=0) and H 2 (Δv=2), while that for D 2 is O2(1Δg,v′=0→3Σg-,v″=1) and D 2 (Δv=2), which entirely matches the calculation by Kustrev.