The first implementation of the rotating shadowband spectroradiometer allows the routine collection of direct normal solar spectral irradiance data at 512 wavelengths. These data are used on clear days to calculate spectra in optical depth. A persistent absorption band near 477 nm in the first optical depth spectra collected led to the eventual identification of six bands in the 400 to 1080 nm range resulting from absorption by O2 collision pairs. All of these O2-O2 bands were discovered earlier using atmospheric long pathlength or laboratory high pressure techniques, however, it is notable that this method's sensitivity permitted their detection at one air mass. These data suggest that there are no unexplained narrowband absorption features in the visible/near-infrared, that excess optical depth measured near 1000 nm may now have an explanation, and that the linearity of the O2-O2 absorption may be exploited to investigate mean pathlengths in the atmosphere. ¿ 1999 American Geophysical Union