A reaction (actual or hypothetical) in which the types of bonds that are made in forming the products are the same as those which are broken in the reactants, e.g.
I03272-1.png
I03272-2.png
Such processes have advantages for theoretical treatment. The Hammett equation as applied to equilibria [cf. (a)] essentially deals with isodesmic processes.
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1128 (https://doi.org/10.1351/pac199466051077)