The oxygen-donor complexes [U(cp)X3L2][cp =η-C5H5, X = Cl or Br, L = Me3CCONMe2(dmpva) or PPh3O],[U(cp)Cl3L2][L = MeCONMe2(dma)], and [{U(cp)X3L}2][X = Cl or Br, L = Ph2(O) PCH2CH2P(O) Ph2(dppoe)] have been prepared. Products of composition ‘U (cp)2Cl2·dma,’‘U(cp)2X2·dmpva,’ and ‘U(cp)2X2(PPh3O)2’(X = Cl or Br) obtained from [U(cp)X3L2] have been shown to be disproportionation mixtures which include [U(cp)3X] and free ligand, whereas [{U(cp)2X2(dppoe)}2](X = Cl or Br) do not appear to disproportionate to an appreciable extent. The lability of the cp ligand has been utilised to prepare [U(cp)Cl3(PPh3O)2] from [U(cp)3Cl] and [UCl4(PPh3O)2] in the presence of PPh3O. The complexes [U(mcp)Cl3(thf)2], [U(cp)(mcp)2Cl], and [U(cp)Br3(thf)2] have also been prepared (mcp =η-methylcyclopentadienyl, thf = tetrahydrofuran). Infrared and u.v.-visible (solution and reflectance) spectra are reported.