Diet is an important aspect of any society. It reflects complex, inter-linked factors such as status or cultural preference often, within environmental constraints on food production. Studying past diet can, therefore, give the archaeologist new insight into the nature of society. In this study we examine the skeletal remains from Ban Non Wat, northeast Thailand, using dietary isotope analysis in order to better understand patterns of migration and subsistence strategy during prehistory. This site is the most comprehensively excavated in the Upper Mun River Valley, and understanding of prehistoric society here is crucial to answering questions of how social complexity arose in the area. Carbon isotope analysis has highlighted migrant individuals invisible to strontium isotope analysis and shown links between unusual burial practice and differences in diet. We also find that diet changes substantially through time, with more reliance on rice in the Bronze Age, correlated with an increase in social differentiation. There is a move away from reliance on rice agriculture in the Iron Age, a time when oxygen isotopes show that environmental conditions were becoming drier, possibly resulting in rice agriculture becoming less viable.