Vaporisation processes in uranium oxide fuel were investigated under controlled oxidation conditions. A Knudsen cell with a mass spectrometer was adapted to feed an adjustable oxygen flux up to temperatures of 1900 K producing an oxygen pressure in the cell ranging up to 1000 Pa. Experiments were carried out on fresh and irradiated fuels, both pre-oxidised to U3O8 or oxidised online in the Knudsen cell, to measure effects associated with fuel sublimation in the form of different uranium-bearing oxides. Mass spectrometry, electromotive force measurements and thermo-gravimetric analysis were used to determine the vapour equilibrium conditions at fixed free oxygen potentials.