Ethylammonium nitrate (EAN) is a typical protic ionic liquid (PIL) known for a long time. In order to investigate acid-base reaction mechanisms in PIL, thermodynamic quantities of a reaction, which corresponds to autoprotolysis in amphoteric solvents, has been determined in neat EAN. Unlike H3O+ and OH- in water, proton donor and acceptor species in EAN are both neutral; this makes acid-base reaction mechanisms in EAN distinct from that in water. EAN-water mixtures have also been studied.