A large variety of V(V) species has been reported in aqueous solution. At room temperature they depend mainly on vanadium concentration and pH [41]. V(V) is a highly charged cation, so that oxo-anions [VO4] 3 in which vanadium is surrounded by four equivalent oxygen atoms are formed in highly alkaline aqueous solutions (pH 4 14). Protonation occurs as the pH decreases. The formation of snowflake-like or rod-like VO2 in acid V2O5–H2C2O4–H2O system has been reported by us [42,43]. In all these papers, our experiments were carried out with the concentration of vanadium of lg(C)=-0:4318. In present V2O5–N2H4–H2O system, the main existed vanadium specie should be pyrovanadate [V2O7]4_, while larger vanadium specie decavanadate[V10O28]6_ was dominant inV2O5–H2C2O4–H2O system at lower pH. In lower pH, these poly-anions form cycles or chains made of corner-sharing[VO4]3 tetrahedral. Here, the great size and morphology differences of vanadium poly-anions were proposed to explain the formation of different VO2 morphologies.