We have demonstrated the feasibility of developing stable Li+ potentiometric sensors, using a composite plastic technology as electrode support. Both triphylite LiFePO4- and FePO4-based sensors display good Li+ sensing properties (reversibility, stability, linearity in the serum Li+ concentration range). Besides, the reversible insertion of various cations (Na+, K+, Mg2+) into the FePO4 host structure was unraveled, providing the necessary information to clearly demonstrate that, within these materials, the sensing mechanism is nested in the ability of the structure to undergo cation insertion/deinsertion reactions. This process was shown to be governed by the Nernst's law. To our knowledge, besides the LiMn2O4/MnO2 spinel-type system, the LiFePO4/FePO4 constitutes the second example of a sensor based on this mechanism. (C) 2003 Elsevier Science B.V. All rights reserved.