This report describes the first attempt for the development of potentiometric H+ sensors based on liquid polymer electrolytes (PEs). The devices proposed here consist of a reference-polymer electrolyte-glass membrane electrode (R-PEGE) and an indicator electrode. Two types of indicator electrodes were obtained. The first uses a PVC-based polymer membrane as sensitive material, and was used to make an indicator-polymer electrolyte-polymer membrane electrode (I-PEPE). The second was assembled with a sensitive glass membrane, indicator-polymer electrolyte-glass membrane electrode (I-PEGE). Both the reference and indicator electrodes were prepared by the superposition of three layers of different materials, i.e., M|PE|SE|, where M is Mg, Li or Ag, PE is PEG400/(LiCl)(0.349) or PEG400/(delta-MgCl2)(0.112) and SE is a sensitive glass- or PVC-based polymeric membrane. The response of the devices to the H+ concentration (a) is very linear from pH 2 to 13; (b) depends on temperature; (c) is very stable and reproducible; and (d) exhibits response times lower than 40 s. These properties make these devices very promising and indicate the polymer electrolytes as suitable materials for the development of innovative ion selective electrode (ISE) sensors. (C) 2003 Elsevier Science Ltd. All rights reserved.