The synthesis of a series of p-tert-butylcalixI4Iarcne based, electrochemically switchable, ionophores (1), (2), (3), (4) and (5) is described. Their electrochemical properties were investigated in the presence of added Croup I metal perchloratcs. Both compounds (1) and (2) displayed no enhancement of binding for metal ions, due to electrostatic interactions, upon electrochemical reduction. This behaviour is explained on steric grounds, as deduced by molecular modelling. Compounds (3), (4) and (5) all display significant enhancements of metal ion binding upon electrochemical reduction. Compounds (4) and (5) appear to display a high selectivity for sodium ions over other Croup I metal ions. Compound (3) displayed the highest binding enhancements yet recorded for both sodium (3.138 x loS) and potassium (1.398 x 103) ions. Compound (6) (from which (3), (4) and (5) were malic) could be converted to m,my other compounds in which the side arms bear difierent functional groups in an eiiort to control selectivity. For example thio esters or thin arnides for silver, ethyl groups tor potassium.