A subclass of room temperature ionic liquids (RTILs) is protic ionic liquids (PIL) that are composed of onium salts. Acid-base reaction thermodynamics in PILs have been investigated by means of potentiometric and calorimetric titrations. In a typical PIL, ethylammonium nitrate (EAN), autoprotolysis was found to be established, which is generally observed in amphoteric molecular solvents such as water. The thermodynamic parameters of autoprotolysis revealed a drastic change in the reaction mechanism in comparison to that in water. This also affects on the acid-dissociation reaction mechanism of CH₃COOH in EAN, while pKa of CH₃COOH in EAN is not so largely different from that in water due to the enthalpy-entropy compensation. In water-EAN mixtures, thermodynamic parameters of autoprotolysis vary depending linearly on the mole fraction at moderate molar ratio. This might be concerned with nearly ideal mixing of water and EAN. In case of PILs consist of a weak acid and a weak base, the species distribution formed in the mixture was estimated by calorimetric titration. It was revealed that less than a half of Mim (N-methlyimidazole) and CH₃COOH are ionized in an equimolar mixture, forming HMim+ and CH₃COO^- . This combination cannot be classified into a PIL.