Interactions between dissolved organic substances (dissolved organic matter and two model substances), ions (Na+ and Ca2+) and the membrane surface were investigated using a submerged membrane module. The membrane permeability, the fouling formation and the rejection were significantly influenced by the solution's ionic strength and especially by the presence of calcium ions. An increase of the ionic strength led to a decrease of the DOM-rejection of the membrane from 75% to about 50% if Na-ions were added and to about 15% if Ca2+-ions were added. This was due to neutralization of charges of the membrane surface and due to shielding of negative charges of DOM molecules. The ionic strength and pH of the cation solutions were kept at similar values. As the solution ionic strength increases, DOM molecules can permeate more easily through the membrane pores, since their shape and molecular size change. Adsorption of DOM on the membrane surface and in the membrane pores was only observed by filtration of solutions with Ca2+. The results showed that in the presence of Ca2+, adsorption played a mayor role in flux decline and fouling formation.