An electrochemical CO2 gas sensor using Li2CO3 and Li2TiO3 + TiO2 as sensing and reference electrodes, respectively, and Li3PO4 as the electrolyte is the subject of this paper. The sensor response to CO2 gas showed a systematic deviation from the prediction of the Nernst equation at low p(CO2). Based on the electromotive force (emf) measurement, the transference numbers of Li3PO4, a lithium-ion conductor, were estimated for different p(CO2) values, and the conduction domain boundary for Li3PO4 separating n-type electronic conduction from ionic conduction was constructed. The conduction domain predicts that change in the Li activity in the sensing side of the cell drives the Li3PO4 electrolyte to a mixed (n- type electronic and ionic) conduction region at low p(CO2). Hebb-Wagner dc polarization measurements also indicate n- type electronic conduction in Li3PO4 with a mixture of Li2CO3 and gold as a reversible electrode. The transference numbers obtained from both the emf measurement and the Hebb-Wagner polarization measurements demonstrate that the origin of the non-Nernstian behavior of the CO2 sensor is due to the lithium mass transport from the Li2CO3-sensing electrode to the Li3PO4 electrolyte, resulting in nonstoichiometry of Li3PO4 at temperatures above 500 degrees C. (c) 2005 The Electrochemical Society. [DOI: 10.1149/ 1.2129180] All rights reserved.