Solid electrochemical sensors based on sodium super ionic conductor (NASICON) and spinel-type oxide CoCr_2−x Mn_xO₄ (x = 0, 1, 1.2, 1.4 and 2) sensing electrode were designed for sub-ppm H₂S detection. In comparison with other spinel-type oxides, the sensor using CoCr_1.2Mn_0.8O₄ showed the maximum response of 178 mV for 10 ppm H₂S at 250 °C. The sensor displayed good stability for H₂S during the testing period. Moreover, the sensor exhibited excellent selectivity toward H₂S against the other interference gases, such as SO₂, NO₂, CH₄, CO, C₂H₄, H₂ and NH₃. A sensing mechanism related to the mixed potential was proposed for the sensor based on NASICON and oxide electrodes. The effect of sintering temperature had also been investigated.