This work has shown that nitrogen dioxide will selectively remove uranium hexafluoride from a mixture containing the hexafluorides of uranium, molybdenum and tungsten via the formation of nitrylium hexafluorouranate(V). The production of lower uranium fluorides and elemental sulphur or sulphur fluorides from many of the attempted preparations of uranium(VI) thiofluorides was predicted from a thermodynamic study of the stabilities of MC1nF6-n (M = U, Mo, W; n = 1-5), MOnF6-2n' MSnF6-2n (M = U, Mo, W; n = 1 or 2) with respect to decomposition and disproportionation. Sorption of the liquid hexafluorides on various potential sorbents was investigated. Encouraging results were obtained for cryolite. Salts such as ARuF6 (A = K, Rb, NH4) were synthesised by direct interactions between AF and RuF5 in AHF. These reactions were found to require the presence of a polar solvent. It was found that ruthenium pentafluoride in liquid uranium hexafluoride could be removed by the Introduction of trace quantities of water.