Abstract A thermochemical model to describe the chemical state of irradiated nuclear fuel has been advanced and validated by comparison to results of experiments on naturally-enriched UO 2 with additions of selected simulated fission products. These experiments involved controlled oxidation in Ar/H 2O/H 2 gas mixtures conducted at the Atomic Energy of Canada Limited–Chalk River Laboratories. A coulombic titration technique provided measurements of moles of oxygen acquired by the samples in relation to oxygen potential. Emphasis was placed on the role of molybdenum in buffering the oxidation of fuel. This treatment is expected to be especially useful when integrated into fuel performance codes that make use of thermodynamics as boundary conditions in heat and mass transfer computations.