Gases will dissolve in liquids to an extent that is determined by the equilibrium between the undissolved gas and the gas that has dissolved in the liquid (called the solvent). The equilibrium constant for that equilibrium is:
where:
* =  the equilibrium constant for the solvation process
* =  partial pressure of gas  in equilibrium with a solution containing some of the gas 
* =  the concentration of gas  in the liquid solution
The form of the equilibrium constant shows that the concentration of a solute gas in a solution is directly proportional to the partial pressure of that gas above the solution. This statement is known as Henrys law and the equilibrium constant  is quite often referred to as the Henrys law constant.
Henry's law is sometimes written as:
where  is also referred to as the Henrys law constant. As can be seen by comparing equations () and () above,  is the reciprocal of . Since both may be referred to as the Henrys law constant, readers of the technical literature must be quite careful to note which version of the Henry's law equation is being used.
Henry's law is an approximation that only applies for dilute, ideal solutions and for solutions where the liquid solvent does not react chemically with the gas being dissolved.