Crystalline oxodiperoxovanadate(V) complexes containing co-ordinated 2·2′-bipyridine(bipy), 1,10-phenanthro-line(phen), and oxalato-groups, C2O42–(ox), M[VO(O2)2L],nH2O (M = Na, K, or NH4; L = bipy or phen; n= 2–5) and M3[VO(O2)2(ox)],2H2O (M = K or NH4), have been prepared. The complexes are formed in hydrogen peroxide solutions of trioxovanadate(V) ions in the presence of corresponding ligands. They are soluble in water, where molar conductivity measurements indicate the presence of 1 : 1 and 1 : 3 electrolytes, respectively. With large excesses of the bidentate nitrogen ligands, complexes are obtained, of simplest formula LH2[V2O2(O2)4L2],6H2O (L = bipy or phen), which do not contain a monomeric peroxovanadate(V) group. The oxodiperoxovanadate(V) complexes are not analogous nor isomorphous with eight-co-ordinate triperoxoniobate(V) and -tantalate(V) complexes containing the same bidentate ligands and prepared under similar conditions.