Protactinium pentachloride, pentabromide, and oxide tribromide react with an excess of tropolone in nonaqueous solvents to yield tetrakis(tropolonato)protactinium(V) halides of the type Pa(trop)4X (trop = C7H5O2–; X = CI or Br). Attempts to prepare Pa(trop)5 have been unsuccessful. Reactions in ethanol involving lithium tropolonate and the tetrakis(tropolonato)metal(V) halides, or Pa(trop)4 and tropolone, result in the precipitation of Pa(trop)4OEt, which on the addition of hydrochloric acid–acetone mixtures is converted into Pa(trop)4CI. This forms a 1 : 1 adduct Pa(trop)4CI,DMSO with dimethyl sulphoxide but does not react with dimethylformamide. The quadrivalent tropolonates Pa(trop)4, Np(trop)4, and Pu(trop)4 have been prepared by reactions in either aqueous or non-aqueous solvents. Pa(trop)4, but neither Np(trop)4, nor Pu(trop)4 reacts with lithium tropolonate to form a pentakis-complex. Oxidation of Pa(trop)4 in perchloric acid solution yields Pa(trop)4CIO4. Some chemical properties, i.r. spectra and crystallographic results are reported for these compounds and for some quadrivalent thorium tropolonates.