An electrolytic bath is concentrated in situ by a process of electrodecantation during the principal electrolytic process for which the bath is employed, e.g. a coating process, utilizing in the electrodecantation process at least some of the electric current which is used in the principal electroytic process. Alternatively, the composition may be concentrated in the electrolytic bath in a separate stage when the principal electrolytic process is not taking place, but there is then used at least one electrode which is also used in the principal electrolytic process.