After a review of most common sensor principles for the detection of CO 2 with emphasis on solid state potentiometric sensors, the thermodynamic principles of recently developed potentiometric CO 2 sensors with open reference electrodes are briefly discussed. These sensors are based on acid-base interactions and allow sealing-free cell arrangements with signals being independent of the oxygen partial pressure. Results concerning the CO 2 and O 2 partial pressure as well as temperature dependence are presented. At least one of the reference electrodes could be identified to show a very fast response to varying CO 2 partial pressures. Coulometric titrations were performed in order to check the phase stability of the reference electrodes. In this way the TiO 2-rich part of the quasi-binary phase diagram TiO 2-Na 2O is elucidated.