In the case of ceramic membranes with pore diameters lower than 10 nm, it was shown that the electrostatic interactions between charged particles and membrane surface charge is an important parameter for explaining the membrane selectivity. So, we have conducted several studies about cobalt spinel membranes (CoAl 2 O 4 ) fired at 450 and 600°C. The pore size is about 4.3 and 5.1 nm, respectively. Firstly, studies of electrophoretic mobility and surface charge measurements were performed on powder and streaming potential determination was measured on membranes. Secondly, we have measured the rejection rate and we have determined the selectivity of salt solutions with different ionic strengths. A correlation between all the results has been established.