Nanofiltration (NF) membranes retention depends on charge repulsion and size exclusion, combining the properties of ultrafiltration and reverse osmosis membranes. The molecular weight cut-off of two NF membranes was determined using cross-flow filtration of different molecular weights of non-ionized polyethylene glycol (PEG). Their retention of humic acid (HA), fulvic acid (FA), diallyldimethylammonium chloride and copolymer of dimethyl aminoethyl acrylate was also determined. NF270 membrane's pure water flux was higher than NF90. NF270 also produced lower retention compared to NF90. Observed retention of PEG using NF270 was lower than NF90, but real retention of PEG was higher for NF270 than NF90. This unexpected result is due to the inconsistency of the concentration polarization model. According to the concentration polarization model, real retention determination is dependent on permeate flux (J(v)) instead of taking account of membrane flux reduction. The two membranes are supposed to operate at similar flux to overcome this inconsistency. Incomplete retention of poly diallyldimethylammonium chloride (PDADMAC) and copolymer of dimethyl aminoethyl acrylate (CoAA) is due to the presence of their monomers. HA retention is higher than FA because of its higher molecular weight range. NF270 membrane retention of HA and FA is lower than NF90 because of its higher pore size and porosity. Solute retention is constant with increasing pressure due to the competition between concentration polarization and dilution effects at the studied range of trans-membrane pressure (TMP).