Protactinium pentaiodide, Pal5, is best prepared by direct union of the elements at 400–450° in a vacuum, but, if protactinium is not available, the pentaiodide is obtained in good yield by the reaction of silicon tetraiodide with protactinium pentachloride, pentabromide (both at 150–200°), or pentoxide (600°). Protactinium oxyiodides, PaOl3 and PaO2l, are obtained by reaction of the pentaiodide with antimony trioxide at 150°in vacuo or, less satisfactorily, by the action of oxygen on the pentaiodide. The oxytri-iodide disproportionates above 450° in a vacuum to yield the volatile pentaiodide and a residue of the dioxymonoiodide. The mixed bromo-iodide PaBr3l2 is obtained when the component pentahalides react together at 350°. Some chemical properties, X-ray powder diffraction results, and infrared spectra are reported for these compounds and for the hexaiodoprotactinate(V), Ph3MeAsPal6, which is formed by reaction of the component halides in non-aqueous solvents. Further infrared studies are also reported for the protactinium(V) oxybromides.