The protactinium pentafluoride hydrates PaF5,2H2O and PaF5,H2O are obtained from reactions between vacuumdried protactinium(V) hydrous oxide and gaseous hydrogen fluoride at 40 and 60°, respectively. Diprotactinium(V) oxide octafluoride, Pa2OF8, is obtained from this reaction at 140° and is the ultimate product when solutions of protactinium(V) in aqueous hydrofluoric acid are evaporated to dryness in vacuo. Pa2OF8 decomposes when heated in the atmosphere at 220–290° to yield protactinium(V) dioxide monofluoride, PaO2F, which is converted into triprotactinium(V) heptaoxide monofluoride, Pa3O7F, at 560–580°. At intermediate temperatures a series of structurally related solid solutions are obtained whilst above 650° Pa2O5 is formed. PaO2F appears to be identical to the phase originally believed to be β-Pa2O5; Pa3O7F is isostructural with high-temperature U3O8. These reactions, and others involving hydrofluorination or fluorination of various protactinium(V) oxide halides, are discussed together with some chemical properties, i.r. spectral results, and X-ray powder diffraction results.