This work focused on the role and effect of adsorbents on natural organic matter (NOM) removal and fouling control in ultrafiltration (UF) membrane systems for water treatment. Two adsorbents, ferrihydrite (FH) and powdered activated carbon (PAC) were compared in terms of the degree of fouling control under the condition where the same amount of dissolved organic carbon (DOC) was removed. The influence of adsorbent cake layers on water permeability was investigated by comparing the UF performance in the presence and absence of adsorbents dosed. Although the same level of DOC removal efficiency was achieved by the two adsorbents, the reduction in fouling varied significantly depending on the size of membrane pores employed. It was found that adsorbent-specific interaction with NOM molecules having multi-dispersed, heterogeneous property may be responsible for different fouling behaviors in ultrafiltration water treatment. The attachment of NOM to PAC particles led to an increase in the cake layer resistance, whereas the reverse happened with NOM-laden FH particles. Scanning electronic microscopy was not able to exhibit the difference in the degree of fouling during UF of lake water that was pretreated with different adsorbents. The membrane fractionation of lake water NOM molecules employed to characterize their size distributions confirmed which portion of NOM present in lake water was removed preferentially by adsorbents, leading to fouling reduction. This helped to interpret and control the fouling behavior when adsorbents were added to the membrane processes. (C) 2010 Elsevier B.V. All rights reserved.