A study of rice farmers in an irrigated area in Central Thailand shows significant differences in productivity and incomes between tenant farmers and owners, even though average farm sizes and input levels are comparable. These differences can be attributed primarily to differences in land quality. A change from sharecropping to a leasehold tenancy has been induced by the Act of Legislation on Land Tenure passed in 1974. Landowners manage to capture benefits of new technologies and irrigation by renting out poorer quality land and farming better quality land themselves. Market access tends to widen resulting productivity and income differentials. The stated aims of land tenure reform legislation have not been achieved.