Three ionic liquids (ILs), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylimidazolium trifluoromethanesulfonate, and 1-ethyl-3-methylimidazolium tetrafluoroborate, were used for the development of electrochemical NO2 sensors and electrochemical transistors. NO2 sensors with a solid electrolyte containing an IL showed good stability, reversibility, and sensitivity up to 365 pA/ppb. The output characteristics of the transistors exhibited suitable properties with respect to the drain-source current vs. the drain-source voltage and the gate-source modulation. Concerning the dynamic characteristics, the fastest response of the drain-source current from I-ON to I-OFF was similar to 2 ms. The time constant depends on the type of the cations and the material of the source and drain electrodes.