We report a stable carbon isotope study of two species of extinct baboons, Papio robinsoni and Theropithecus darti, dating approximately to 1.7-1.9 my, from the Member 1 deposits at Swartkrans cave, South Africa. ¹³C/¹²C ratios were measured using tooth enamel carbonate as sample material, after appropriate chemical pretreatment. The results for the fossil baboons, compared with those of known browsers and grazers from the same member, show that P. robinsoni had a C₃-based diet, while T. darti was mainly dependent on C₄ grasses. The study shows that carbon isotope analysis of tooth enamel clearly distinguishes graminivorous and C₃-based diets in fossil primates.