Ion exchange equilibria Cl- - NO3- Cl--SO42-, NO3--SO42- on fibrous strong base anion exchangers on the basis of graft copolymer of styrene and divinylbenzene to polypropylene fibers with groups of benzyl trimethyl- (TMA), triethyl- (TEA), and tributyl- (TBA) ammonium have been studied and characterized by values of the equilibrium coefficients and separation factors. They were compared with the same equilibria on ion exchange resins Dowex I with different cross-linkage. An equation allowing calculating the separation factor for different ionic concentrations and ratios of the ions in solutions of uni-and divalent ions from the equilibrium coefficient invariant to the concentration has been derived. The selectivity of nitrate and chloride sorption increases with increasing size of alkyl radicals in the functional group and with increasing percent of divinylbenzene in the resins. A quantity characterizing selectivity of ion exchanger to bivalent ion against the univalent ion named concentration of zeroth selectivity (CZS) has been suggested to describe the efficiency of ion exchangers in processes of water purification from univalent ions, specifically nitrates in waters containing sulfates. At concentrations above the CZS the univalent ion is absorbed selectively compared to the bivalent one; the "dumping effect" in frontal chromatograms of mixtures of such ions is not possible above the CZS point and this criterion can be used for choosing the ion exchanger for selective removal of univalent ions from mixtures with polyvalent ones. The CZS values for conventional strong base ion exchangers in ion exchange NO3--SO42- is higher than typical total ionic concentrations in drinking waters (1-3 x 10(-3) eq/L). For Dowex 1 x 10 it was found equal 1.3 x 10(-2) eq/L. For the fibrous ion exchanger with benzyltributylammonium groups CZS = 3.3 x 10(-6) eq/L. This ion exchanger is suitable for selective removal of nitrates from the solutions with concentrations higher than this value and has exceedingly high selectivity to nitrates which makes the process of its regeneration difficult. The most suitable fibrous ion exchangers for application in drinking waters purification from nitrates with guarantied absence of the nitrate dumping appeared FIBAN A-1 TEA having the CZS values 4.3 x 10(-4) eq/L. (c) 2007 Elsevier Ltd. All rights reserved.