The Osagean-Meramecian rocks in the western part of the Illinois Basin include crinoidal carbonates and shale of the Fern Glen, Burlington-Keokuk, and Warsaw Formations. In the southern and eastern Illinois Basin, the succession comprises siliceous limestone of the Fort Payne Formation and crinoidal-bryozoan carbonates of the Ullin Limestone. These units are capped by shallow-water carbonates of the Salem, St. Louis, and Ste. Genevieve Limestones. Temporal and lateral relationships of some of these units are poorly constrained. This study is the first attempt at using carbon isotope data to help resolve some of the existing ambiguities in the correlation and lateral relationship of these units despite their lithologic differences. Results show that carbonate d13C values are generally more negative (< 2.5‰) throughout the Fort Payne and overall become progressively more positive upward, increasing to > 3.5‰ at the Ullin–Salem boundary. There is also a corresponding change in lithology from a siliceous-crinoidal-bryozoan carbonate succession below to a shallow-water, oolitic, and peritidal lithofacies above. Although less dramatic, a similar trend is also noted in the Fern Glen, Burlington-Keokuk, and Warsaw in western Illinois. Here, d13C value is approximately 2.0‰ at the base of the Fern Glen and gradually increase to > 3.5‰ at the base of the Salem. Isotope data further reveal that this overall trend also includes a more positive excursion in the Fern Glen and Burlington-Keokuk (d13C mostly between 3.0 and 3.5‰) and a negative excursion in the lower Warsaw (~ 1.0 to 1.5‰). Similar excursions can be seen in the Fort Payne in southern Illinois, which displays a positive excursion of approximately 2.2‰ in the lower part, followed by a negative excursion at 1.0 to 1.5‰ in the upper part. This result suggests that the fossiliferous shale of the lower Warsaw in western Illinois may be laterally equivalent to that in the upper part of Fort Payne in southern Illinois. The Fern Glen–Burlington-Keokuk succession, however, appears to be equivalent to the lower part of the Fort Payne. The negative d13C trend may be related to the effect of oceanic upwelling, the influx of d13C-depleted terrestrial carbon, or both. Either scenario could have provided ample nutrients, causing a proliferation of echinoderms and bryozoans.