This study shows that alternating polyelectrolyte deposition on porous supports can yield nanofiltration membranes that allow high water flux along with selective ion transport. Membranes composed of 4.5−5 layer pairs of poly(styrene sulfonate)/poly(allylamine hydrochloride) (PSS/PAH) on porous alumina allow water fluxes of 1−2 m3 m-2 day-1 at 4.8 bar while exhibiting MgSO4 rejections of 96%. Rejections of CaCl2 and Na2SO4 depend on polyelectrolyte deposition conditions and the composition of the outer layer of the membrane. In general, divalent-ion rejection increases when the charge of the outer layer of the membrane has the same sign as the divalent ion being rejected. Increasing the concentration of the supporting electrolyte present during deposition of the terminating PSS layer of PSS/PAH membranes results in a higher surface charge, and hence higher Na2SO4 rejections (up to 95%). Nanofiltration with mixed solutions of NaCl and Na2SO4 yields Cl-/SO42- selectivities of about 30 when the top layer of ...