In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H⁺) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid, is deprotonation.) Some examples include

• The protonation of water by sulfuric acid:

  H₂SO₄ + H₂O ⇌ H₃O⁺ + HSO⁻
  ₄

• The protonation of isobutene in the formation of a carbocation:

  (CH₃)₂C=CH₂ + HBF₄ ⇌ (CH₃)₃C⁺ + BF⁻
  ₄

• The protonation of ammonia in the formation of ammonium chloride from ammonia and hydrogen chloride:

  NH₃(g) + HCl(g) → NH₄Cl(s)

Protonation is a fundamental chemical reaction and is a step in many stoichiometric and catalytic processes. Some ions and molecules can undergo more than one protonation and are labeled polybasic, which is true of many biological macromolecules. Protonation and deprotonation (removal of a proton) occur in most acid–base reactions; they are the core of most acid–base reaction theories. A Brønsted–Lowry acid is defined as a chemical substance that protonates another substance. Upon protonating a substrate, the mass and the charge of the species each increase by one unit, making it an essential step in certain analytical procedures such as electrospray mass spectrometry. Protonating or deprotonating a molecule or ion can change many other chemical properties, not just the charge and mass, for example solubility, hydrophilicity, reduction potential, and optical properties can change.