The Department of Atomic Energy envisages the use of thoria based fuel in the third phase of nuclear power generation. The fuel will consist of solid solution of thorium-uranium and thorium-plutonium in the form of their oxides. The former will contain 2.5 mole % UOz while the latter about 4 mole % PuOz. Since no other country in the world has used such fuel, no data is available on its behavior under long-term irradiation. The high temperature chemistry of fuel can however provide some insight into the behavior of such fuel during irradiation and could be of considerable help in the assessment of its long-term integrity. The high temperature chemistry of the fuel essentially involves the measurement of thermodynamic properties of the compounds formed in the multi-component systems comprising the fuel matrix, the fission products and the clad. The physical integrity of the fuel under long-term irradiation can be predicted with the help of basic thermodynamic data such as the Gibbs energy of formation of various compounds and their thermophysical properties such as thermal conductivity and coefficient of thermal expansion derived from experimental measurements. The paper highlights the measurements made on some typical systems relevant to the prediction of thoria based fuel behaviour during long-term irradiation. The experimental problems faced in such measurements are also discussed.