The streaming potential and permeate flux of polyethersulfone organic membranes were determined during the filtration of solutions having various compositions. The permeate flux is only marginally affected by pH variations or the nature and concentration of species in solution. Streaming potential measurements for membranes having 20 and 100 kDa cut-off values give isoelectric points of respectively 2.4 and 2.2. The specific adsorption of divalent ions leads to a decrease in the streaming potential for a given ionic strength and pH. The zeta potential was determined from previous results using the Helmoltz-Smoluchowski relation. It leads to results that are incompatible with the classic theory because this relation does not take into account the double layer overlapping which occurs in the pore. Corrections made to take into account this phenomenon lead to more satisfactory results.