CO2-gas sensors were fabricated using NASICON-type dense ceramics with new compositions in the Na3Zr2-(X/4)-Si2-xP1+xO12 system. The bulk ceramics were prepared from sol-gel-processed powders. Dense electrolytes were obtained for compositions with x = 0.667 and x = 1.333. An improved sinterability was observed with respect to NASICON with conventional composition, which was attributed to liquid-phase sintering. The CO2-gas sensors using these dense samples showed a stable electromotive force (emf) response in dry atmosphere that was similar to the theoretical Nernstian value for a two-electron electrochemical reaction. This was observed also for the NASICON sample with composition x = 1.333 that showed a conductivity far lower than that of the NASICON with conventional composition. The emf changed quickly with changes of the CO(2-)gas concentration, and steady-state values were observed. The response time, in adsorption and desorption of CO2, was very fast, especially at high CO2 concentrations. The influence of humidity on the CO2-sensing performance was investigated. A lower sensitivity and slower response were obtained in humid CO2 gas, especially at low CO2 concentrations. CO2-sensing measurements at various gas-flow rates were performed to evaluate the reactions occurring at the measuring electrode. The occurrence of a side reaction on the measuring electrode was observed, i.e., the formation of sodium oxides. In dry gas, the reaction took place at low CO2 concentrations and small now rates, whereas the reaction was strongly enhanced in humid environments and occurred over the entire CO2 concentration range. However, the sensor performance recovered after switching from humid gas to dry gas. This demonstrated that the humidity affects the emf because of the Na2Ox formation at the electrode, and, thus, the solid electrolyte itself was not degraded by humidity.