The Henry adsorption constant is the constant appearing in the linear adsorption isotherm, which formally resembles Henrys law; therefore, it is also called Henrys adsorption isotherm. It is named after British chemist William Henry. This is the simplest adsorption isotherm in that the amount of the surface adsorbate is represented to be proportional to the partial pressure of the adsorptive gas:
where:
* X - surface coverage,
* P - partial pressure,
* K - Henry's adsorption constant.
For solutions, concentrations, or activities, are used instead of the partial pressures.
The linear isotherm can be used to describe the initial part of many practical isotherms. It is typically taken as valid for low surface coverages, and the adsorption energy being independent of the coverage (lack of inhomogeneities on the surface).
The Henry adsorption constant can be defined as:
where:
*  is the number density at free phase,
*  is the surface number density,