The aquation of [W₂O₄(edta)]^2–(edta = ethylenediaminetetra-acetate) to W₂O₄²⁺(or a closely related species) proceeds to completion in HCl 2 mol dm^–3. First-order kinetics have been observed, with the rate constant (1.16 × 10^–4 s^–1 at 25 °C) in 2 mol dm^–3 HCl some 60 times less than for the corresponding reaction of [Mo₂O₄(edta)]^2–. With HClO₄ 2 mol dm^–3 aquation is accompanied by oxidation to W^VI. No aquation (or oxidation) has been observed with HClO₄ < 0.5 mol dm^–3, under which condition redox reactions of the edta complex could be investigated. At 25 °C, I= 0.50 mol dm^–3(H–LiClO₄) rate constants for the first stage of the 1 : 2 oxidations of [W₂O₄(edta)]^2– with [IrCl₆]^2– and [Fe(phen)₃]³⁺(phen = 1,10-phenanthroline) are 6.3 × 10⁵ dm³ mol^–1 s^–1 and >10⁷ dm³ mol^–1 s^–1 respectively. For the [IrCl₆]^2– reaction activation parameters are ΔH^‡= 5.4 kcal mol^–1 and ΔS^‡=–13.8 cal K^–1 mol^–1 respectively. Rate constants (dm³ mol^–1 s^–1) for other oxidants, I= 0.10 mol dm^–3, are [Fe(CN)₆]^3–(0.058), [Co(bipy)₃]³⁺(2.0)(bipy = 2,2′-bipyridine), and I₃^–(ca. 20). The complex [W₂O₄(edta)]^2– is a much stronger reducing agent than [Mo₂O₄(edta)]^2–.