The high sensitivity absorption spectrum of the a1Δg-X3∑g- band of 16 O 17 O, 17 O 18 O and 17 O 2 has been recorded by CW-Cavity Ring Down Spectroscopy near 1.27μm. The spectra were obtained between 7640 and 7917cm −1 with a 17 O-enriched sample at room temperature and at 80K. Due to the I=5/2 nuclear spin of the 17 O atom, the nuclear hyperfine structure of the transitions could be partly resolved at low temperature. The parameter coupling of the nuclear spin to the electronic angular momentum in the a 1 Δ g state has been derived for the three studied isotopologues of oxygen.