Esters can be hydrolyzed to carboxylic acids under acidic or basic conditions. Base-promoted hydrolysis of esters is a nucleophilic acyl substitution reaction in which esters react with an aqueous base, followed by an acid to give carboxylic acids. This reaction is also known as saponification because it forms the basis for making soaps from fats.

The reaction requires a base in stoichiometric amounts, which participates in the reaction and is not regenerated later. So, the base acts as a reactant and not as a catalyst.

In the first step of the base-promoted hydrolysis mechanism, the hydroxide ion attacks the carbonyl carbon of the ester to form a tetrahedral intermediate. In the second step, the carbonyl group is re-formed with the elimination of an alkoxide ion. The strongly basic conditions result in deprotonation to give a carboxylate ion and alcohol. After the saponification is complete, an acid protonates the carboxylate ion to give the carboxylic acid.

Isotope-labeling studies using an ester labeled with ¹⁸O isotope support the mechanism. After saponification, all the oxygen of the alcohol is replaced by the oxygen isotope (¹⁸O). None of the oxygen isotopes (¹⁸O) appeared in the acid. This indicates that the reaction occurs by cleaving the acyl-oxygen bond rather than the alkyl-oxygen bond. As a result, one of the oxygens in the carboxylate ion comes from the nucleophilic hydroxide ion.

The saponification reaction is irreversible, as the carboxylate ion is unreactive towards nucleophilic substitution.