The high-resolution absorption cross-section measurements of O2 in the 83.0--83.5 nm spectral region have been carried out with a resolution of 0.0008 nm and at temperatures of 295 K and 535 K. These are apparently the first such high-temperature cross-section measurements of O2 in this wavelength band. The absorption features of the O2 I--X (4,0) band systems are observed to be quite diffuse with a line width of full width at half maximum (FWHM) equal to 0.03 nm. Therefore the cross-section values of O2 in this spectral region will not critically depend upon the resolution as long as it is better than 0.03 nm, as is evident in the reported data. As the temperature increases from 295 K to 535 K we observed an increase of 30% in the cross-section values on both sides of the triplet features of the I--X (4,0) band, whereas a 20% decrease occurs over the peak region. ¿ 2000 American Geophysical Union