Sulfate reduction in the sea-water end member of mixing-zone fluids changes the fluid-saturation indices for carbonate minerals. Mixing-zone fluids were modeled quantitatively using Atlantic sea water and a fresh-water end member from Barbados for sulfate reduction in the presence of goethite, in the absence of goethite, and with back-oxidation of aqueous sulfide to sulfate. Minor amounts of sulfate reduction enhance dissolution of CaCO 3 in brackish to saline mixing-zone fluids, whereas major amounts of sulfate reduction enhance precipitation in all mixing-zone fluid compositions. Secondary reactions produce either CaCO 3 dissolution or precipitation, which depend in magnitude on the amount of sulfate reduction. Dissolution results from the back-oxidation of aqueous sulfide to sulfate, and precipitation occurs from the transformation of iron oxides to iron sulfides. pH buffering, due to sulfate reduction, prevents a linear correlation of calcite-saturation indices with the moles of calcite necessary to dissolve or precipitate to reach equilibrium. Without iron sulfide precipitation or the back-oxidation of aqueous sulfide, the moles of calcite that can be dissolved continue to increase up to about 25% sulfate reduction, even though the degree of undersaturation begins to decrease with greater than 5% sulfate reduction. The effects of iron sulfide precipitation and the back-oxidation of aqueous sulfide are greatly enhanced by increasing the amount of sulfate reduction, being generally more than 0.1 g of calcite per kg of water dissolved or precipitated, respectively, at 50% sulfate reduction.