Bulk type solid state potentiometric CO2 sensors were fabricated based on Li2CO3-BaCO3 binary auxiliary phase as a sensing electrode, Li3PO4 mixed with 5 mol% SiO2 as a solid electrolyte and Li2TiO3 mixed with 10 mol% TiO2 two phase system as a reference electrode. The concentration of BaCO3 was varied in the auxiliary phase (mol%). The electromotive force (emf) of the investigated cells was linearly dependent upon the logarithm of CO2 partial pressure over a wide range of 500-5000 ppm. The sensors have shown good CO2 sensing properties at low concentrations of Barium carbonate additive. 10 mol% BaCO3 loaded Li2CO3 sensor has shown the highest absolute emf and a very close theoretical Delta emf/dec value at 500 degrees C than the other counterparts. The slopes of emf as a function of logP(CO2) were in good accordance with the Nernstian slopes based on a two electron electrode reaction between the temperatures 450-500 degrees C. During the heat treatment, the interface formed between the sensing electrode and electrolyte and revealed by SEM, EDAX and X-ray diffraction studies indicates a new glassy phase consisting of Barium silicate (Ba4Si6O16) and Barium phosphate (Ba3P4O13). The penetration or diffusion of the Barium components deep inside the interfacial layer along with rich Li2O content was proposed to be responsible for the improved sensing characteristics of the sensor. (c) 2006 Elsevier B.V. All rights reserved.