Measurements of the rejection of perchlorate anion (ClO4-) have been performed by using two thin-film composite nanofiltration (NF) membranes and four ultrafiltration (UF) membranes. The latter four membranes are all from the same manufacturer and, ostensibly, from the same material family. These were chosen to systematically change the membranes steric properties, while keeping the same material chemistry, thus, the enthalpic interactions should stay constant. The perchlorate anion (at a concentration of 100 g/L of ClO4- by "spiking" with KClO4) was presented to the membrane as a pure component, in binary mixtures with other salts, and at varying pH and ionic strength (conductivity). Also, a natural source water was "spiked" with perchlorate anion and used to document the effects of a complex mixture, including natural organic matter, on the observed rejection. All filtration measurements were performed at approximately the same permeate flow rate to minimize artifacts from mass transfer at the membrane interface. In general, the results indicate that, in a pure component system, target ions (in this case ClO4-) can be significantly excluded from like-charged membranes with pores large with respect to the size of the ion, but this rejection capability decreases in the presence of a sufficient amount of other ions that can screen the electrostatic force field.