This research is aimed at improving our understanding of membrane processes for the removal of organic contaminants in potable water treatment installations. Experimental observations are reported on the deposition of humic acid and PAC (Powdered Activated Carbon) on UF membranes and their removal by backwashing. PAC deposition is monitored by employing on-line optical microscopy techniques, and membrane fouling by flux measurements. Effects related to the operating mode (cross-flow vs. batch) and physicochemical conditions (PH, ionic environment, membrane hydrophilicity) are investigated. It is observed that deposition of PAC alone has a minor effect on membrane flux; however, PAC removal efficiency by backwashing is also small. Furthermore, PAC deposition is significantly reduced in the presence of humic acid. Cross-flow is associated with reduced deposition and fouling and improved humic acid rejection. The most notable effect of increasing pH in the range 6 to 8 is in increasing humic acid rejection. Significant effects are observed in the presence of calcium, which is known to form complexes with the humic acid and increase its tendency for aggregation. The amount of deposits increases sharply with calcium concentration but fouling (reflected in the flux measurements) reaches a maximum and then decreases when aggregation is fairly rapid, at relatively high concentrations. Backwashing removes the humic acid deposits and partially restores flux but it is not completely efficient under all physicochemical conditions especially with the less hydrophilic membrane.