Tonga is a Polynesian island chain that was initially colonized by the Lapita culture about 2700 years ago. Its inhabitants went on to found the Tongan State about 700 years ago. Our project uses carbon and nitrogen stable isotope analyses to investigate and compare the diets of Lapita and Tongan State individuals. Sampled burial locations include the Talasiu shell midden cemetery dating to the Late Lapita period as well as four burial mounds on Tongatapu, Tonga, thought to date to the Tongan State period (700-100 BP) and two isolated burials. New bone collagen ?13C and ?15N results were obtained from 65 individuals and analyzed along with previously published carbon and nitrogen stable isotope results. Fifteen new radiocarbon dates on bone from five locations within Tongatapu are also reported. We show that low to moderate diagenesis in our samples tends to increase ?13C but does not have a directional bias for ?15N. Isotopic data that pass diagenesis checks strongly support a substantial marine component within the Late Lapita diet in Tonga, which is similar to the early Lapita diet at Teouma but differs from other Late Lapita data. Stable isotope and radiocarbon evidence indicate that Tongan State individuals had a more strongly terrestrial diet, and that elite and non-elite diets were similar during the Tongan State. We also show that the transition to a more terrestrial diet may have happened near the start of the Tongan State period.