NASICON Na 1þx Zr 2 Si x P xÀ3 O 12 ; 0 x 3 ð Þ powders were prepared by solid-state synthesis, a fraction was ion exchanged by Li or K, verified by X-ray diffraction and energy dispersive X-ray analysis, blended with tin oxide powder, transferred to thick film pastes, and dispensed on a commercial sensor substrate (Heraeus, Germany). Simultaneous gas sensitivity measurements on nine SnO 2 /NASICON-type (M 1þx Zr 2 Si x P xÀ3 O 12 , x=0, 2.2, 3, M = Li, Na, K) composites in thermocyclic sensor operation mode by exposure to different concentrations of Ethanol, Toluene, Propylene, CO, and H 2 in humidified synthetic air show a strong correlation of the gas-sensing properties with the mobile ion concentration and type of the solid electrolyte additive.