Different Stat proteins are activated through phosphorylation of unique tyrosine residues in response to different cytokines and growth factors. Interferon-γ activates Stat1 molecules that form homodimers and bind cognate DNA elements. Here we show that treatment of permeabilized cells with 200- 500 μM peroxo-derivatives of vanadium, molybdenum, and tungsten results in the accumulation of constitutively phosphorylated Stat1α molecules. In contrast, treatment of permeabilized cells with orthovanadate, vanadyl sulfate, molybdate, and tungstate at the same range of concentrations does not result in the accumulation of activated Stat1α molecules in the absence of ligand. However, these compounds inhibit the inactivation of interferon- γ-induced DNA-binding activity of Stat1α. A 4-6-h exposure of the permeabilized cells to orthovanadate, molybdate, and tungstate, but not vanadyl sulfate, results in a ligand-independent activation of Stat1α, which is blocked by the inhibition or depletion of NADPH oxidase activity in the cells, indicating that NADPH oxidase-catalyzed superoxide formation is required for the bioconversion of these metal oxides to the corresponding peroxo-compounds. Interestingly, ligand-independent Stat1α activation by peroxo-derivatives of these transition metals does not require Jak1, Jak2, or Tyk2 kinase activity, suggesting that other kinases can phosphorylate Stat1α on tyrosine 701.