We report the acid–base property of typical protic ionic liquid, 1-alkylimidazolium bis(trifluoromethanesulfonyl)amide ([CnImH+][TFSA−], n: alkyl-chain length), investigated by potentiometric titration. The equilibrium constant, Ks = [CnIm][HTFSA] on the autoprotolysis reaction (CnImH+ + TFSA− ⇄ CnIm + HTFSA) was successfully determined for n = 2 and 4 systems using an ion-selective field effect transistor (IS-FET) electrode. The pKs values for n = 2 and 4 were estimated to be 12.3 and 12.6 mol2 dm− 6, respectively, indicating that the autoprotolysis reaction is almost independent of the alkyl chain length. Temperature dependence of the potentiometric titration was also performed to obtain the reaction enthalpy. The enthalpy value, 41 kJ mol− 1, obtained here is appreciably smaller than that for ethylammonium nitrate (83 kJ mol− 1).