Time‐resolved spectra of the decay of the O2 infrared atmospheric ( a¹Δg → X³Σg−) ( 0,0) band emission in the evening twilight at 41.7°N have been obtained using the Fourier transform infrared (FTIR) technique. The measurements were made during a two‐month summer‐fall period which included the autumnal equinox. The late twilight variations were well described by single exponentials with decay time constants which ranged from (44.4±1.6) min in early August to (61.1±2.1) min near the equinox. In this connection, a strong positive trend in the time constant was observed during late August and early September. When combined with available data for collisional quenching and for fall‐winter, upper‐mesospheric altitude distributions of O2 (a¹Δg, ν=0) in the late twilight, these observations support a radiative lifetime of about one hour.