Tetronasin is a recently discovered ionophoric antibiotic that, apart from its interaction with sodium ion. can be used to construct potentiometric sensors selective to calcium ion. The observed response is super-Nernstian and dependent on pH, as it is expected from a mixed hydrogen-calcium equilibrium, with optimal response conditions at pH 5.0 (sensitivity 65 mV/decade). The selectivity pattern to common inorganic cations is determined. The studies with primary ion and interferents, validate an optimal formulation of a PVC membrane for Ca2+ which employs 1% ionophore, 0.2% anionic sites, 66% o-nitrophenyloctylether and 33% PVC. The interaction with Ca2+ is studied through an evaluation of the mechanisms and responses of the constructed membranes by means of Electrochemical Impedance Spectroscopy. Results in the range 50 KHz-0.05 Hz indicate an electrochemical system not impeded by charge transfer or slow diffusion. The single semicircle observed on the corresponding Nyquist plots is translated into a bulk resistivity lower than 700 K ohm cm, with a geometric capacitance of 98 pF (for a 10(-3) M CaCl2 solution). (c) 2005 Elsevier Ltd. All rights reserved.