The present process purifies a salt Cat+X−, where Cat+ is any cation and X− is an organic anion having at least 6 carbon atoms. The salt is an impurity in a composition comprising a water-soluble salt (Cat+)nYn−, where Yn− is an inorganic anion or an organic anion, different from X−, with at most 10 carbon atoms. The process involves converting X− into an acid HX by adding an acid (H+)mZm−, forming two phases: an organic phase comprising HX and an aqueous phase comprising the water-soluble salts (Cat+)mZm− and (Cat+)nYn− or acids (H+)nYn−. The organic phase is separated and the aqueous phase passes over a basic anion exchanger which binds Zm− and Yn− and liberates OH−, giving an aqueous mixture comprising Cat+OH−. The organic phase and the aqueous mixture are converted into Cat+X− and H2O, optionally with addition of further HX.