Nano- or ultrafiltration of inorganic salts in the presence of a polyelectrolyte in the feed solution is investigated in this work. The membranes are completely impermeable to the polyelectrolyte. At low concentrations of polyelectrolyte, a gel layer on the membrane surface is not formed. At such polyelectrolyte concentrations, the concentration of inorganic salt in the permeate stream can be higher than that in the feed solution. This salt concentration effect is the reverse of what is obtained with conventional membrane processes, where the permeate salt concentration is lower than, or equal to, the salt concentration in the feed solution. It is shown that, in the nano- or ultrafiltration of inorganic salts in the presence of a polyelectrolyte, the ratio of the salt concentration in the permeate to that in the feed is improved when the initial salt concentration in the feed solution is low. Concentration polarization has a negative impact on this concentrating effect. A theory that elucidates the observed phenomenon is presented, together with experimental data for potassium chloride solutions and NTV-dimethyl-N-2-propenyl-2-propen-1-aminium chloride homopolymer filtered on an ultrafilter with a pore size of 5 nm.