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Chemisorption, or chemical adsorption, is a process in which a gas or liquid phase species adheres to a solid surface through the formation of a chemical bond. This bonding involves sharing or transfer of electrons between the adsorbate and the substrate, causing substantial changes in the electronic structure at the surface. Chemisorption energies are typically much larger than those of physisorption and often require an activation energy, making the process more temperature-dependent and usually resulting in only submonolayer or monolayer coverage.

In contrast to physisorption, which is governed by weak van der Waals forces and is generally reversible at moderate temperatures, chemisorption involves stronger bonds such as covalent or ionic interactions. The adsorbate often becomes chemically bonded to a specific site on the surface, which can alter reactivity, catalysis, or surface properties. Because of the bond formation, chemisorption often proceeds more slowly and can be partially irreversible under practical conditions, although some systems can reversibly adsorb and desorb species.

Mechanistically, chemisorption may involve electron transfer, hybridization of orbitals, or formation of surface complexes. Kinetics are influenced by surface structure, defect density, temperature, pressure, and the nature of both the substrate and the adsorbate. Modeling chemisorption commonly requires approaches that account for activation barriers and site-specific interactions, rather than simple multilayer isotherms used for physisorption.

Characterization techniques include temperature-programmed desorption, X-ray photoelectron spectroscopy, infrared spectroscopy, and vibrational spectroscopy, often complemented by microscopy or surface-structural methods. Applications span heterogeneous catalysis, gas sensing, corrosion science, and hydrogen storage, where chemisorption underpins surface reactions and binding behavior.