Incomplete utilization of reactive ferric iron minerals by dissimilatory iron-reducing bacteria has been ascribed to inhibition by adsorption of Fe2 + to the mineral or bacterial surfaces. We tested this hypothesis by monitoring the reduction of 6-line ferrihydrite and amorphous ferric phosphate by Shewanella putrefaciens for up to 50 hours, at circumneutral pH and in the presence of excess electron donor. The microbial iron reduction incubations were performed with Fe(III) substrates or bacteria that had been exposed beforehand to solutions containing dissolved Fe2 + (range: 0-500 M) for relatively short durations (≤12 hours). The reduction rates were not, or only weakly, affected by pre-sorbed Fe2 +. A slight decrease in the reduction rate was observed for ferric phosphate particles whose ferrozine-extractable Fe2 + concentrations approached the estimated mineral surface site density. Thus, under the range of experimental conditions considered here, sorbed Fe2+ did not limit the transfer of electrons from membrane-bound reductases to Fe(III) centers at the mineral surface. We hypothesize that adsorption of Fe2 + inhibits the initial Fe(III) reduction kinetics only when the surface sites of the mineral surface, or the metal binding sites in the bacterial cell wall, are (nearly) all saturated with Fe2 +.