Since 1990 the organo-mineral concept has been used to develop ultrafiltration membranes which resulted in the Zirfon (R) membranes. These membranes are polysulfone based membranes with zirconia as inorganic filler material and are prepared by a phase-inversion technique. The use of high polymer contents in the casting solution resulted in a tight ultrafiltration membrane (cut-off value 3200). As zirconia is an amphoteric oxide the existence of a membrane charge was expected. In this work the salt retention of this membrane is studied. Filtration experiments were carried out with different salts (Na2SO4, CaCl2, MgSO4) at Various pH values (2-10). The retention for MgSO4 was low and showed a minimum at pH 3. At pH 10 the retention for Na2SO4 was 58%, whereas for CaCl2 no retention was found. By lowering the pH the Na2SO4 retention decreased, whereas the CaCl2 retention increased to 33% at pH 2. All experimental results are in accordance with the Donnan exclusion theory and show an amphoteric behaviour of the membrane with an IEP at pH 3. At a higher pH value the membrane charge is negative, at a lower pH value the charge is positive. The effect of ion concentration is illustrated for NaCl and for Na2SO4. The retention of salt mixtures was also studied. For a mixture of NaCl and MgSO4, it was found that a separation of chloride and sulphate is possible, but only at low concentrations.