In so much as most solid electrolytes and mixed ion/electron conductors are generally specific to the migration of one ion, solid state electrochemical cells have applications as electrochemical filters. In the present paper several examples are considered from the author’s research where filtering as an electrochemical phenomenon occurs. Oxide ion conducting electron/ion conductors are taken as a specific example, and ways to optimize oxygen transport through such materials are discussed. Examples are given from corrosion research where selective ion migration through oxide films formed on alloy surfaces can lead to alloy enrichment in the sub-surface region. Finally, an electrochemical cell is described where selective filtering of cations from a nonstoichiometric sulphide electrode via a solid electrolyte can release anions to enhance specific reactions.