The mechanism of green rust sulfate (GR-SO4) formation was determined using a novel in situ approach combining time-resolved synchrotron-based wide-angle X-ray scattering (WAXS) with highly controlled chemical synthesis and electrochemical (i.e., Eh and pH) monitoring of the reaction. Using this approach,GR-SO4 was synthesized under strictly anaerobic conditions by coprecipitation from solutions with known Fe-II/Fe-III ratios (i.e., 1.28 and 2) via the controlled increase or pH. The reaction in both systems proceeded via a three-stage precipitation and transformation reaction. During the first stage,schwertmannite (Fe8O5(OH)(4.5)(SO4)(1.75)) precipitated directly from solution at pH 2.8-4.5. With increasing pH ( >5), Fe2+ ions adsorb to the surface of schwertmannite and catalyze its transformation to goethite (alpha-FeOOH) during the second stage of the reaction. In the third stage, the hydrolysis of the adsorbed Fe2+ ions on goethite initiates its transformation to GR-SO4 at pH >7, The GR-SO4 then continues to crystallize up to pH similar to 8.5. These results suggest that with an Fe-II/Fe-III ratio of <= 2 in the initial solution the structural Fe-II/Fe-III of the GR-SO4 will be close to that of the starting composition.