Auto-protolysis (self-ionization) of water is a chemical reaction in which two water molecules react to produce a hydroxide ion (OH–) and a hydronium ion (H3O+).

The reaction involved can be represented as: 

H2O(I) + H2O(I)       H3O+(aq)       + OH-(aq)
                                   Hydronium ion        Hydroxide ion 

Auto-protolysis of water indicates its amphoteric nature i.e., its ability to act as an acid as well as a base. The acid-base reaction can be written as:

H2O(I)    +     H2O(I)                H3O+(aq)       +         OH-(aq)
(Acid)            (Base)                     (Conjugate acid)       (Conjugate base)