Separation of sulphuric acid from copper sulphate at high salt and acid concentrations was possible with flat-sheet NF membranes at 20 bar. The NF45, NF270 and Desal-5DK membranes retained copper sulphate effectively (96-98% copper retention). Sulphuric acid retention remained low for all the studied membranes. Donnan forces were present and negative retention of sulphuric acid was observed when solutions that contained concentrated copper salt and dilute acid were filtered. As the total ion concentration and the ratio of acid/salt increased, the effect of the salt on acid retention decreased. The partial dissociation of sulphuric acid and copper sulphate decreased the effects created by the Dorman equilibrium. Experiments indicated that when concentrated acid solutions are nanofiltered, the separation is mainly determined by size effects and Donnan effects are marginal.