The previously discovered phenomenon of improvement of the retention of one electrolyte upon addition of another one has been studied both theoretically and experimentally. The phenomenon has been shown to be caused by the so called "decompensation" of convective and electric transmembrane flows of counter ions brought about by the difference in mobilities of counter ions of the resulting electrolyte mixture. It has been shown both theoretically and experimentally (for supported membranes of sulfonated polysulfone) that the same physics accounts for not only the improvement of the retention of more mobile counter ions upon addition of less mobile but also for a significant deterioration of the retention (dawn to a negative one) of less mobile counter ions upon addition of more mobile. Experimental correlations of the phenomenon with the ratio of mobilities of counter ions, the concentration of starting electrolyte and transmembrane pressure difference have been in complete agreement with theoretical predictions.