It is shown theoretically and with simulated data that O(lS) lifetimes determined by the cross-spectral method (Paulson and Shepherd 1965) are significant overestimates. A comparison is made of the cross-spectral and impulse function analysis (Burns and Reid 1984) methods using photometric data collected at Macquarie Island (54. 50S, l59�00E geographic). The results support the view that the O(lS) state is excited predominantly by an indirect process.