The main emphasis of this study is the difference in behaviour between those two organic types. Stirred cell experiments were used to investigate charge and size effects in the rejection behaviour of bulk natural organics as well as low molecular mass (LMM) acids by nanofiltration membranes. To distinguish between size and charge effects thorough characterisation of membrane, natural organics and ionic environment was carried out. Membrane zeta potential was determined as a function of the ionic environment and the intrinsic membrane rejection was measured with dextran, ionic solutions and synthetic surface water. The behaviour of three thin film composite (TFC) and one cellulose acetate (CA) membranes was studied as a function of solution chemistry (pH, ionic composition and strength, calcium concentration, and organic type). Rejections of DOC, UV254nm, as well as the cations calcium and sodium were measured. Natural organics in selected feed and permeate samples were also characterised using liquid chromatography organic carbon detection (LC-OCD) which allows the characterisation of the organic fractions in the low concentration permeates and hence give insight into the rejection of low molecular mass (LMM) organics. The retention of such LMM compounds is important due to their impact on the microbiological regrowth potential of the product water The results emphasised that charge and size are both important for cations and low molecular mass acids. While the bulk of the natural organics are retained due to size exclusion that is independent on solution chemistry for the membranes investigated, the retention of LMM acids follows different mechanisms. The analysis of organic fraction in the permeates showed that the rejection of low molecular mass acids is strongly pH, and thus charge, dependent while this effect of solution chemistry on the LMM acids was masked when only bulk organic rejection was measured.