Permeation of mixed salt solution through charged membrane is a complex process, and depending upon electrostatic interaction among different types of ions and wall charges, complex separation behaviors such as negative rejection of a salt in the mixture have been observed for such systems. Negative rejection of a salt in an aqueous mixture of salts has been observed for nanofiltration membranes that have very small pores (pore diameter similar to 1 nm). We report in this work studies on separation of mixed salt solutions of NaCl and KCl using a charge ultrafiltration membrane of large pore size (pore diameter 15-92 nm) whose preparation is described elsewhere [Workneh, S.; Shukla, A. J. Membr. Sci. 2008, 309, 189]. At a low total concentration of electrolytes in the feed (0.01 M), when the effective exclusion of co-ions occurs inside the membrane, negative rejection of NaCl is obtained. It is found that irrespective of the composition of the salt mixture used in this work, the magnitude of negative observed rejection increases with pressure initially but beyond a certain pressure its magnitude decreases by a small amount. It is also found that as the NaCl composition is increased the magnitude of negative observed rejection first increases but beyond a value of 60% NaCl it decreases for all the pressure differences used in this work. Simulated values of the intrinsic rejection coefficient, however, are found to increase monotonically with pressure, indicating that the maximum in magnitude of observed negative rejection is due to concentration polarization. With variation in feed composition the intrinsic rejection shows a trend similar to the observed rejection.