Tailings from a gold mine containing 1000 mg/kg of As were used to predict the release of As over an extended period of time. Post-cyanide mine processes were aiming to form arsenates and Fe-hydrates for effective As-immobilization. Speciation of the As in ore and tailings samples revealed that mining processes have dissolved the majority of the arsenopyrite in the ore, causing secondary As phases to co-precipitate with newly formed Fe-hydrates. Weathering Cell Tests (WCT) were conducted to assess the effect of weathering on the stability of As in the tailings. As-bearing Fe-hydrates remained intact during the early stages of the WCT. During later stages of the WCT, the release of As, Fe and S increased due to pyrrhotite oxidation and the destabilization of As-bearing Fe-hydrates. Low proportions of As was released in WCT, but additional pyrrhotite oxidation as pH falling to < 3 could further destabilize As-bearing Fe-hydrates