ABSTRACT: Carbon isotopic studies conducted on the upper Paleocene (Thanetian) and lower Eocene (“Sparnacian”—Ypresian) of the Paris Basin allow us to place the lithostratigraphic succession of the Mont Bernon Group in a spatial and temporal framework. The d13C (negative) carbon isotope excursion (CIE) associated with the Paleocene/Eocene Thermal Maximum (PETM) in marine sections at the NP9a/b calcareous nannoplankton zonal boundary and also in terrestrial stratigraphies, occurs in the Mortemer Formation, below the Vaugirard Formation. Moreover, a characteristic isotopic signature with stratigraphically successive negative and positive peaks permits a refinement in correlation betweeen the diverse members of the Mont Bernon Group. The strongly positive values (in upper Zone NP9) are found in the Meudon Member (Conglomérat de Meudon). These results imply that: (1) the Meudon Member constitutes the base of the Vaugirard Formation, but not the base of the Sparnacian “Stage”; (2) the mammal fauna of the Meudon Member (=late Biochron NP9) is somewhat younger than that of the basal Wasatchian North American Land Mammal Age (NALMA; which is stratigraphically correlative with, or even slightly older than, mid-Biochron NP9); (3) the base of the Sparnacian “Stage” of the Paris Basin is slightly older than the base of the Wasatchian NALMA and only marginally younger than the Chron C24r/C25n boundary. Moreover, the carbon isotopic signature suggests correlation of the Reading 1 beds/facies of the London Basin with the Mortemer Formation and Reading 2 beds with the lower part of the Vaugirard Formation of the Paris Basin. With the recognition that the Sparnacian is confined to the ca. 0.85 m.y.-long stratigraphic interval between the CIE at its base and the FAD of Tribrachiatus digitalis at the base of the Ypresian, and that it is inappropriate to lower the base of the Ypresian Stage by about 1m.y., we recommend (re)insertion of the Sparnacian Stage as the basal stage of the Eocene Series.