The interaction of humic substances on organic and biofouling during the treatment of seawater using direct contact membrane distillation (DCMD) was studied. Organic fouling was investigated at different feed temperatures in terms of distillate flux pattern and detailed organic characterization using liquid chromatography-organic carbon detection. The penetration of organics through the membrane pores was observed with SEM cross-section analysis. Also, chemical cleaning analysis showed seawater organic fouling was irreversible in DCMD. Humic substances and low molecular weight (LMW) organics are dominant organic contents in seawater. In the tested DCMD system, humic substances were thermally disaggregated to LMW-humic organics. This phenomenon was more significant in the presence of salt (NaCl), while inorganic scalant (CaSO4) reduced the disaggregation of humic substances due to the binding effect. In this study, the assimilable organic carbon (AOC) test was used to estimate the biological fouling potential during the MD process. AOC concentration is closely associated with biofilm growth in water and on the membrane (biofouling). The AOC concentration increased as the concentration of LMW-humic organics increased. This indicated the possibility of biofouling increasing in the feed and on the membrane in the MD process. (C) 2014 Elsevier B.V. All rights reserved.