In chemistry, autoprotolysis is a molecular autoionization, a chemical reaction in which a proton is transferred between two identical molecules, one of which acts as a Brønsted acid, releasing a proton that is accepted by the other molecule, which acts as a Brønsted base. Any chemical that contains both acidic hydrogen and lone pairs of electrons to accept H+ can undergo autoprotolysis.

For example, water undergoes autoprotolysis in the self-ionization of water reaction.

For example, ammonia in its purest form may undergo autoprotolysis:

Another example is acetic acid: