Abstract: In a gas sensor element, an alkali metal ion conductor is provided with a detection electrode comprising an alkali metal carbonate in dissociation equilibrium with CO2 gas on one side thereof and a solid standard electrode on the other side thereof. The alkali metal carbonate is bonded to the alkali metal ion conductor via a bonding agent comprising an alkaline earth metal carbonate which is capable of forming an eutectic low-melting composition with the alkali metal carbonate.

Abstract: A CO.sub.2 gas sensor using a solid reference electrode in combination with a lithium ion conductive solid electrolyte which is at dissociative equilibrium with CO.sub.2 gas, characterized in that the solid reference electrode is made of a lithium containing, mixed transition metal oxide conductor which is of the same conduction type as the solid electrolyte. Further, the solid electrolyte comprises a complex carbonate containing LiAlO.sub.2 as an aggregate. Furthermore, a mixed electrode is used for the solid-electrolyte type gas sensor, which is prepared by the steps of: mixing a Au powder or paste with a powder of an oxide excluding Al.sub.2 O.sub.3 ; rendering a resultant mixture to a paste; applying the paste onto a solid electrolyte; and sintering the applied paste.