Tunable NO2-sensing characteristics using Ni1-xCoxO-sensing electrodes (SEs) at high temperature are reported. Various compositions of cobalt (Co)-doped NiO solid solutions (Ni1-xCoxO, 0 < x < 0.3) were synthesized by the citrate-gel method, and each of the Ni1-xCoxO samples obtained was utilized as SE for yttria-stabilized zirconia (YSZ)-based planar sensor for detection of NO2 at 800 degrees C under wet condition. As a result, the NO2 sensitivity increased with increasing cobalt content (x) in Ni1-xCoxO, and the maximum sensitivity was observed for Ni0.9Co0.1O. The sensitivity of the sensors was almost invariant to the change in water vapor concentration in the sample gas. Moreover, the sensor exhibited excellent long-term stability for about 6 months. Interestingly, the sensor attached with Ni0.8Co0.2O SE showed excellent selectivity to NO2. The effect of Co substitution in NiO on the rates of electrochemical reactions at the Ni1-xCoxO/YSZ interface and the catalytic activity for the gas-phase conversion of NO2 to NO was evaluated, and the sensing mechanism of the present sensor was discussed.