The validity of using carbon isotopes in biological apatite from bone for dietary studies has been questioned because of diagenetic alterations and supposed exchange of carbon from the burial environment. A laboratory experiment subjected a sample of bone to a synthetic groundwater whose carbon isotopic composition had been adjusted to +9750per thousand. Analyses indicate that simple carbonates exchanged rapidly with the groundwater, but that the biological apatite exchanged very little, if any, of its carbon, even with two years exposure. A natural experiment involved analysis of mammoth and mastodon teeth that had been submerged in sea water for 11,000 years. With proper cleaning, the original dietary signals were well-preserved despite the long exposure to oceanic dissolved carbonate of much different isotopic composition. These experiments suggest that properly cleaned biological apatite is an appropriate analytical phase for dietary studies on bone or teeth as old as 10, 000 years.