The removal of various levels of iron and manganese along with chlorine dosages from lake water was investigated using different ultrafiltration (UF) systems in conjunction with an in-line prechlorination step. In particular membrane fouling, caused by oxidized iron and manganese particles, was assessed in depth with visualization of the membrane surfaces. For feedwater containing 1.0mg/L of Fe and/or 0.5 mg/L of Mn, substantial iron removal was achieved even without the addition of chlorine due to the oxidation of ferrous iron to ferric by dissolved oxygen and the consequent formation of less soluble iron hydroxide particles. Only negligible amounts of manganese removal occurred in the absence of chlorine, but with a dose of chlorine the manganese removal efficiency increased markedly and reached a level of more than 80% (corresponding to less than 0.1 mg/L Mn) at a chlorine dosage of 3 mg/L as Cl-2. With a higher dosage of chlorine (e.g., 5 mg/L Cl-2), there was no significant increase in the removal of metal ions but more serious membrane fouling occurred. Also, oxidized manganese claimed a greater responsibility for membrane fouling during UF with chlorination. This phenomenon was in close association with the kinetics of manganese oxidation and its oxidized particles' deposition inside the pores during backwashing, rather than its accumulation on top of the membrane skin layer. Turbidity and natural organic matter (NOM) removal levels of efficiency were enhanced with the addition of chlorine in the presence of iron and manganese because the metal oxides, created by chlorination, could serve as adsorbents. (c) 2005 Elsevier B.V. All rights reserved.