The impact of the water background inorganic matrix (pH and calcium hardness) on the natural organic matter (NOM) removal by nanofiltration was investigated. Pre-treated surface water and two NOM model Substances of different molecular weight and hydrophilicity (salicyclic acid and purified Aldrich humic acid, 2-3 mg C/L) were nanofiltered using a polypiperazine amide membrane. Results showed that the background inorganic matrix greatly influenced the NOM removal by a negatively charged membrane, regardless of the type of NOM. which Caused no effect on the results. Flux showed no significant variation with running time and recover rate. but it decreased with pH and especially in the presence of calcium. Such results were related to the low DOC concentration and the low flux that minimised the concentration polarisation. and to the pH effect on the membrane and NOM charges. The pH increase caused pore narrowing. larger NOM hydrodynamic radius and stronger repulsions between the negatively charged membrane and NOM functional groups, yielding higher rejections. which in turn. increased the osmotic gradient with a subsequent flux decline. Flux and rejections decreased further in the presence of 1 mM Ca2+ which reduced the membrane negative charge, and decreased the sieving effects and increased the chemical interactions. (c) 2006 Elsevier B.V. All rights reserved.