We present dental enamel stable carbon and oxygen isotope data, histological analyses of daily cross striations and perikymata, and microwear data of Late Miocene primates Indopithecus and Sivaladapis nagrii and an early Pleistocene primate Theropithecus delsoni, known from the Indian Siwaliks. The results indicate that the Late Miocene giant ape Indopithecus had a C₃ diet, likely including nuts, seeds and fruits with hard cover, and most probably lived in a seasonal forest that experienced long dry and wet seasons. While the seasons changed as depicted by intra-annual variation in δ¹⁸O values, the C₃ component of diet was similar throughout the year, as indicated by consistent δ¹³C values. Sivaladapis nagrii dental enamel δ¹³C and δ¹⁸O values, striations and pit percentages indicate that this small primate (3 kg) was a C₃ mixed folivore-frugivore. Theropithecus delsoni dental enamel δ¹³C and δ¹⁸O values and microwear pattern indicate a diet comprising mainly C₄ grasses or sedges.