Four polyoxometalate (POM)-based Cu complexes with the hydroxylated pyridine analogue 3-(2-hydroxylpyrid-4-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl (btpo), H-3{[Cu-2(btpo)(2)](PW12O40)}center dot 2H(2)O (1), H-3{[Cu-2(btpo)(2)](PMo12O40)}center dot 2H(2)O (2), H-2{[Cu-2(btpo)(2)](SiW12O40)center dot SO4 (3), and H-4{[Cu-4(btpo)(4)](SiW12O40)}center dot 8H(2)O (4), were synthesized hydrothermally under acid conditions. Single-crystal X-ray structural analysis reveals that 3-(pyrid-4-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl was hydroxylated into btpo in compounds 1-4, again providing structural evidence for the Gillard mechanism, in which H2O as a weak nucleophile can attack the alpha-C atom of N-heterocyclic molecules at a lower pH value (ca. 1.0) with the help of POMs.