Cavity ring-down spectroscopy (CRDS) has been applied to the detection of oxygen atoms, on the highly forbidden D-1(2) <-- P-3(2) line at 630.030 nm. Results are presented for CRDS detection in a discharge flow system, in which the atoms are prepared by a microwave discharge of N2O/Ar or O-2. Comparison of concentrations determined by CRDS and chemical titration by NO2 is made. CRDS is found to be a non-intrusive technique for the determination of oxygen atom concentrations in the range of 10(14) atoms cm(-3) and higher, with an estimated accuracy of less than or equal to20%. (C) 2004 Elsevier B.V. All rights reserved.