This paper considers nanofiltration (NF) of concentrated organic/inorganic mixtures using the FILMTEC™ NF-200B membrane. Mixtures of salt (up to 17% (w/v)) and lactic acid (2% (w/v)) were used as model solutions. The work centres on the effects of salt concentration, pH and temperature on the flux and rejection of lactate. For all solutions under study, the rejection of salt was low, while the rejection of lactate was maximal at neutral pH, and decreased with salt concentration and temperature. The flux was found to decrease with salt concentration and increase with temperature, the activation energy being higher for low fluxes. The flux for pure water and 2% (w/v) lactic acid was at a maximum at neutral pH, but for salt-containing solutions, it increased with pH in the whole range analysed (pH 3–10). The observed flux and rejection patterns suggest that the effects of skin shrinkage in concentrated salt solutions, and sorption of lactate by the membrane, affect behaviour in addition to the conventional effects of charge, solute size and osmotic difference between the retentate and permeate streams.