Protactinium pentachloride is best prepared by reaction of low-fired or vacuum-dried pentoxide with thionyl chloride vapour at 375–425° or, mixed with carbon, with chlorine and carbon tetrachloride vapour at 400–500°. Diprotactinium(V) oxyoctachloride, Pa2OCl8, which occurs as a by-product of the latter reaction, is also obtained by heating the pentachloride with the stoicheiometric quantity of oxygen at 400°, It disproportionates above 270° in a vacuum, forming the pentachloride and the non-volatile diprotactinium(V) trioxytetrachloride, Pa2O3Cl4, which in turn disproportionates above 520° in a vacuum to form PaCl5 and protactinium(V) dioxymonochloride, PaO2Cl. By varying the conditions, the reaction between PaCl5 and oxygen can also be made to yield pure samples of diprotactinium(V) trioxytetrachloride, and an unstable oxytrichloride, PaOCl3, has also been observed. Some chemical properties, X-ray diffraction studies, and infrared spectra are reported for these compounds.