Equations have been derived to describe the open-circuit potentiometric response of a p-type conducting polymer membrane, The final model is derived step by step from the model of a single-metal electrode by using the concept of solvated electron and from the model of a solid-state ion-selective and/or electron-selective electrode containing a sparingly soluble metal-rich salt in the membrane. All electrodes considered, i.e. the metal, ion-selective and polymer electrodes, are presented as systems based on an ionic salt of the M+e- type. The ionic and redox sensitivities of a p-type conducting polymer are described by assuming two possible equilibration pathways. In the first approach the ion-electron exchange process at the polymer surface without a change of the oxidation state of the polymer is discussed and in the second the metathesis of the polymer surface with a concurrent change in the oxidation state of the polymer is considered. The equations derived allow us to interpret the anionic, cationic and redox sensitivities and to discuss the observed changes in the standard potentials.