The first atmospheric profiles of the ultraviolet/visible (UV/vis) absorption bands of the collision complex O2-O2, or O4 in brief, are reported. The O4 absorption profiles are inferred from direct Sun spectra observed from the LPMA/DOAS (Laboratoire Physique Mol¿culaire et Application/Differential Optical Absorption Spectroscopy) balloon gondola. Seven O4 absorption bands - centered at ~360.7, 380.2, 446.7, 477.1, 532.2, 577.2, and 630.0 nm - are investigated for atmospheric pressures (p) ranging from ~500 hPa to ~40 hPa and temperatures (T) ranging from 203 K to 250 K. For the encountered atmospheric conditions, it is found that, (a) the band shapes do not change with T and p and (b) the peak collision pair absorption intensities (&agr;i) concurrently increase with decreasing T (by about 11 % over a ΔT=50 K). That result is in agreement with previous laboratory O4 studies mostly conducted at high O2 partial pressures (up to several hundred bars). Furthermore, by reasonably assuming that the O4 absorption cross sections are T-independent, the inferred T-dependence of &agr;i(T) suggests a thermally averaged enthalpy change ⟨ ΔH ⟩ = -(1207¿83) J/Mol involved in the formation of O4. Our inferred ΔH is in reasonable agreement with the orientation and spin averaged O4 well depth De(O4) (= -(1130¿80) J/Mol) measured in a recent O2-O2 collision experiment, when accounting for the rovibrational energy change during O4 formation (186 J/Mol). ¿ 2001 American Geophysical Union