A Ca2+ sensor based on the admittance change of synthetic membranes supported on derivatized silicon electrodes is reported. The ion-sensitive membranes consist of mixed monolayers of phospholipid and the Ca2+-ligand ETH 1001. The electrodes are characterized electrochem. and their sensor performances are described. Through ion complexation by the neutral ligand, elec. charges are incorporated in the low-dielec.-const. environment of the sensitive layer. The no. of charged complexes and thus the membrane admittance depend on the ion content of the electrolyte. The membrane response to Ca2+ and the competitive binding of other cations are studied. A noncompetitive process is obsd. if several ionophores, each one specific for a different ion, are incorporated in the membrane. Mixed layers with tetradecyl-18-crown-6 and ETH 1001 show both K+ and Ca2+ sensitivity. These electrodes allow direct comparison with a previously reported K+ sensor based on tetradecyl-18-crown-6. [on SciFinder (R)]