The carbon isotope ratio (delta (super 13) C) of plant material is commonly used to reconstruct the relative distribution of C (sub 3) and C (sub 4) plants in ancient ecosystems. However, such estimates depend on the delta (super 13) C of atmospheric CO (sub 2) (delta (super 13) C (sub CO2) ) at the time, which likely varied throughout Earth history. For this study, we use benthic and planktonic delta (super 13) C and delta (super 18) O records to reconstruct a long-term record of Cenozoic delta (super 13) C (sub CO2) . Confidence intervals for delta (super 13) C (sub CO2) values are assigned after careful consideration of equilibrium and non-equilibrium isotope effects and processes, as well as resolution of the data. We find that benthic foraminifera better constrain delta (super 13) C (sub CO2) compared to planktonic foraminiferal records, which are influenced by photosymbiotes, depth of production, seasonal variability, and preservation. Furthermore, sensitivity analyses designed to quantify the effects of temperature uncertainty and diagenesis on benthic foraminifera delta (super 13) C and delta (super 18) O values indicate that these factors act to offset one another. Our reconstruction suggests that Cenozoic delta (super 13) C (sub CO2) averaged -6.1 + or - 0.6ppm (1sigma ), while only 11.2 million of the last 65.5 million years correspond to the pre-Industrial value of -6.5ppm (with 90% confidence). Here delta (super 13) C (sub CO2) also displays significant variations throughout the record, at times departing from the pre-Industrial value by more than 2ppm. Thus, the observed variability in delta (super 13) C (sub CO2) should be considered in isotopic reconstructions of ancient terrestrial-plant ecosystems, especially during the Late and Middle Miocene, times of presumed C (sub 4) grassland expansion.