New water-soluble homoleptic peroxo complexes and heteroleptic peroxo-polyaminocarboxylato (PAC) complexes of tantalum(V) have been prepared. In the case of the peroxo-PAC complexes, the synthesis in the presence of excess H2O2 leads to the oxidation of the nitrogen atoms of the ligand into N-oxides. The compounds correspond to the general formula (gu)(3)[Ta(O-2)(2)(LO2)].xH(2)O (gu = guaniclinium, L = edta or pdta) in which H4LO2 refers to the bis(N-oxide) derivative of the PAC ligand. The Ta-V complexes have been characterized on the basis of elemental and thermal analysis and by IR and C-13 and N-15 NMR spectroscopy. These last two spectroscopic methods have been used to suggest the coordination mode of the PAC ligand in the complexes. ESI mass spectrometry measurements have also been carried out for the peroxo-PAC compounds. The crystal structures of the homoleptic tetraperoxotantalate, (gu)(3)[Ta(O-2)(4)] (1), and the heteroleptic complex, (gu)(3)[Ta(O-2)(2)(edtaO(2))].2.32H(2)O.0.68H(2)O(2) (2b), have been determined, showing, for both cases, an 8-fold-coordinated Ta atom surrounded either by four bidentate peroxides or by two peroxides and one tetradentate edtaO(2) ligand.