New water-soluble bimetallic peroxo-tartrato complexes of niobium(V) and/or tantalum(V) have been prepared, characterized from the structural and spectroscopic point of view, and used as molecular precursors for Nb-Ta mixed oxides. Two new homometallic complexes, (gu)(5)[Nb-2(O-2)(4)(tart)(Htart)].4H(2)O (1a) and (gu)(6)[Ta-2(O-2)(4)(tart)(2)].4H(2)O (2a), and the corresponding heterometallic complex, (gu)(5)[NbTa(O-2)(4)(tart)(Htart)].4H(2)O (3), have been obtained. The crystal structures of the homometallic compounds, (gu)(5)[Nb-2(O-2)(4)(tart)(Htart)].6H(2)O.1H(2)O(2) (1b) and (gu)(6)[Ta-2(O-2)(4)(tart)(2)].6H(2)O (2b), have been determined, showing, for both cases, two 8-fold-coordinated metal atoms, each surrounded by oxygen atoms belonging to two bidentate peroxides, two monodentate carboxylato, and two alkoxo groups from both bridging tartrato ligands. The coordination polyhedron around each metal atom is a dodecahedron. The thermal treatment of complexes la, 2a, and 3 in air at 700 or 800 degreesC, depending of the Ta content, provided Nb2O5, Ta2O5, and the solid solution TaNbO5, respectively. The thermal treatment of a 1:1 Nb/Ta molar ratio mixture of la and 2a has also been studied. BET and SEM measurements have been carried out and reveal these oxides possess relatively high specific surface areas and display a porous character. Comparison between the use of homo- and heterometallic precursors is discussed.