The reaction of VOBr2 with N,N,N'N'-tetrakis(2-pyridylmethyl)ethylenediamine (tpen) and NaH2PO4 (2:1:2 molar ratio) in an aqueous/ethanolic solution yielded purple crystals. X-ray crystal structure analyses revealed that the purple complex was a dinuclear vanadium(IV) complex bridged by a diphosphate anion, [((VO)-O-IV)(2)(mu-P2O7)(mu-tpen)]center dot 10H(2)O. This indicates that a condensation reaction occurred between two phosphate ions during the reaction to form a phosphate phosphate bond. A probable mechanism is proposed for the condensation reaction of two phosphate anions.