The objective of this study was to investigate the retention of phosphate anions, H2PO4- and HPO42-, by nanotiltration. The first part of this study deals with the characterisation of the NF200 membrane used in permeation experiments with aqueous solutions of neutral organic and charged inorganic solutes. In the second part the effects of feed pressure, ionic strength, concentration and pH on the retention of phosphate anions were investigated. Results show that the membrane is negatively charged, its pore radius is around 0.5 nm and the retention order for the salts tested was R(Na2SO4) > R(NaCl) > R(CaCl2). The retentions of phosphate anions are in the order of 85% for H2PO4- and 96% for HPO42-. They are relatively high when compared to retentions of other anions with the same charge. The retentions of phosphate anions, particularly the monovalent species, depend on the chemical parameters (feed concentration, ionic strength, and pH) and applied pressure. The experimental data were analysed using the Speigler-Kedem model and the transport parameters, i.e., the reflection coefficient (sigma) and solute permeability (P-s) have been determined. (c) 2007 Elsevier B.V. All rights reserved.