Cavity ring down spectroscopy has been used to detect the weak a 1 Δ g -X 3 Σ g - electronic transition in oxygen and to obtain a measurement of the radiative lifetime of the a 1 Δ g state. (0,0) band transitions near 1274 nm were scanned using the frequency-doubled output of a tunable OPO system. Pressure-broadened linewidths were measured, and a FWHM-broadening coefficient of 5.5(5) MHz/Torr was obtained. Analysis of the ring down traces provided a measurement of the line strength of an individual rovibronic transition. The Einstein A-coefficient of the a 1 Δ g state was calculated using branching ratios determined from the Honl-London factors. The A-coefficient obtained is 2.3(3)x10 - 4 s - 1 , corresponding to a radiative lifetime of 4300(500) s (~72min). This value is in agreement with those reported by Badger et al. (2.58x10 - 4 s - 1 ) and Spalek et al. (2.24(40)x10 - 4 s - 1 ). In addition, a value of 2.77(8)x10 - 4 5 cm 5 /molecule was obtained for the background collisional absorption coefficient for oxygen at 1274 nm.