The kinetics and mechanism of reductive dissolution of Fe(III) oxides were examined in pure, batch cultures of Pseudomonassp. 200. Primary factors controlling hematite dissolution kinetics were mineral surface area (or concentration of high-energy surface sites), ligand concentration, and cell number. In the presence of nitrilotriacetic acid (NTA), saturation kinetics were apparent in the relationship governing reductive dissolution of hematite. A kinetic expression was developed in which overall iron-reduction rate is functionally related to the concentrations of both NTA and Fe(III).Addition of NTA resulted in a 20-fold increase in the microbial rate of mineral (reductive) dissolution. Mechanisms in which NTA served as a bridging ligand, shuttling respiratory electrons from the membrane-bound microbial electron transport chain to the metal center of the iron oxide, or accelerated the departure of Fe(II) centers (bound to ligand) from the oxide surface following reduction have been postulated. Experimental results indicated that cell-mineral contact was essential for reductive dissolution of goethite.