The D18OLE project aimsat developing the determination of δ18O values of triterpenols in order to reconstruct pastatmospheric O2(O2atm) δ18O values and evaluate changes in the Dole Effect over the Cenozoic. Cultivation of Panicum miliaceumwith varying H2O and CO2δ18O valuesshowed that oxygen in miliacin, a C3-oxygenated pentacyclic triterpene,derives from respired O2, in agreement with biogeochemical observationsand consistant with modeled isotope fractionation factor. δ18O values of triterpenols preserved in the fossil record should thus allow estimating the long-term evolution of O2atmδ18O values.The first challenge consists in affordinga robust determination of sterols and triterpenols δ18O values by GC-Py-irMS. The low O/C ratio for these compounds constitutes an analytical challenge. First results acquired on miliacin and friedelin display large peaks by gas chromatography, preventingacceptable reproducibility. A second issue is the current lack of international lipids standards with known δ18O values.The fractionation factor between O2atmand O in triterpenols will be determinedby cultivating vascular plants and algae under controlled atmosphere with known O2δ18O values. Finally, δ18O values of triterpenols will be determined in a set of lignites and lacustrinesediments that cover two major climatic crisis of the Cenozoic : the Paleocene-Eocene Thermal Maximum and the Eocene/Oligcene boundary.