Iron fouling characteristics of gas permeable composite and wetted microporous hollow fiber membranes were studied. Membrane modules, operated in the scaled end configuration, were pressurized with oxygen and enclosed within a tubular shell. A solution consisting of tap water and ferrous sulfate was metered though the shell side of the module. The added mass-transfer resistance due to iron fouling was determined from the required increase in oxygen feed pressure to the membranes in order to maintain constant flux across the membranes, and was evaluated as a function of operating time and process parameters. Process parameters investigated include Reynolds number, ferric and ferrous iron concentrations, iron oxidation rate, and oxygen flux. It was found that iron oxidation rate, ferric iron concentration, and oxygen flux had no impact on the rate of membrane fouling, whereas the rate and maximum extent of membrane fouling increased with Reynolds number and ferrous iron concentration. The induction period was also found to increase with Reynolds number. A maximum fouling resistance of 2779 s/cm was measured at 17 It for a Reynolds number of 9000 and ferrous iron concentration of 14.0 mg/l. This is 7.2 times greater than the overall mass-transfer resistance for the clean membrane. For all conditions in this study, constant flux across the membrane was maintained, and the impacts of fouling compensated for by operating at higher driving forces. For membrane gas transfer systems where the driving forces can be controlled, such as membrane oxygenation, driving forces can be used to compensate for increases in mass-transfer resistance. However, for applications where driving forces cannot be as easily manipulated, such as VOC stripping, iron fouling is likely to have significant impact on transfer performance.