The flux decline of reverse osmosis and nanofiltration membranes was investigated under constant pressure conditions during humic acid fouling tests. For a given membrane type under a given feedwater composition, increasing pressure resulted in increased flux reduction and foulant accumulation. A limiting flux seems to exist beyond which the membrane flux cannot be sustained. Membranes with initial fluxes greater than the limiting flux experienced severe fouling and their pseudo stable fluxes approached the limiting flux. Flux reduction was much milder when the initial flux was lower than the limiting flux. Furthermore, the limiting flux seems to be independent of membrane properties, probably due to the dominance of foulant--deposited-foulant interaction upon complete foulant coverage over membrane surfaces. On the other hand, strong dependence of the limiting flux on the feedwater composition was observed. The limiting flux was reduced at higher proton, calcium, and/or background electrolytes concentrations, likely due to reduced electrostatic repulsion under these conditions.