The acidophilic heterotrophic eubacterium Acidiphilium SJH was shown to catalyze the reductive dissolution of a wide range of ferric iron?containing minerals (akageneite, goethite, jarosite, natrojarosite, and amorphous ferric hydroxide) and of the mixed ferrous/ferric mineral magnetite. The specific rates of dissolution varied with the structural stabilities of the minerals, such that amorphous ferric hydroxide was the most rapid and jarosite and akageneite were the slowest of the minerals tested. The reductive dissolution of both amorphous ferric hydroxide and magnetite was faster in pH 2.0 than in higher pH (2.8?3.0) cultures, even though Acidiphilium SJH has a pH optimum close to pH 3. Contact between bacteria and ferric mineral was not necessary for reductive dissolution to occur. Adding EDTA or diethylenetriamine pentaacetic acid to bacterial cultures accelerated the solubilization of goethite and amorphous ferric hydroxide. Although cell-free spent media and heat-killed Acidiphilium SJH also appeared to enhance mineral dissolution (indicated by formation of soluble Fe3 + ), this was far less extensive than that in active bacterial cultures, and no iron reduction was observed in the absence of viable cells. Experimental results suggested that Acidiphilium SJH accelerates the reductive dissolution of ferric iron minerals by way of an indirect mechanism, in which bacterial reduction of soluble ferric iron causes a shift in equilibrium between solid phase (mineral) and soluble ferric iron, thereby causing further dissolution of the mineral.