Chemistry Department, Faculty of Science, Mansoura University, Egypt Manuscript received 1.9 October 1990, revised 6 April 1992, accepted 6 August 1992 Several new dioxouranium(VI) complexes of substituted alipatic thiosemicarbazides derived from acetoyl, propionyl. butroyl, isobutroyl valeroyl and isovaleroyl hydrazines and phenyl isothiocyanate, have been synthesised of the general formula [UO2(HL)2], where H2L = tridentate monoanionic thiosemicarbazide. The substituted aliphatic thiosemicarbazides behave in a similar way to tridentate ligands via the amide nitrogen, thioketo and the enolic carbonyl oxygen, with replacement of a hydrogen atom from the latter group. Also, the bond between the thioketo group and uranyl ion is very weak. One of the more striking differences between these complexes and those prepared from substituted aromatic thiosemicarbazide is that the effects of CS in bonding and the absence of the solvent molecules.