The dissolution of several normal and inverse ferrites byClostridiumsp. under anaerobic conditions occurs through enzymatic reductive dissolution of iron (direct action) and through organic acid metabolites (indirect action) produced by the bacteria. The iron in the octahedral coordination in normal spinel and in tetrahe‐dral coordination with the divalent metal in inverse spinel was solubilized by direct action of the bacteria. Although the extent of dissolution of the ferrites differed, there was no clear relationship between the type of ferrite (normal and inverse spinels) and the mechanism of dissolution. The inverse spinels containing cobalt and copper ferrites were solubilized by indirect and direct action, respectively. Among the normal spinels tested, manganese ferrite was solubilized by direct action, while zinc ferrite was solubilized by direct and indirect action. Nickel ferrite was not solubilized by direct or indirect action. Cobalt, copper, manganese, nickel, and zinc ferrites had no effect on the growth of the bacteria. Dissolution of toxic metals and iron from fossil and nuclear energy wastes containing ferrite compounds by anaerobic microbial activity could be significant.