By regarding the process of adsorption as involving a reaction between a molecule of gas and an adsorbing center on the solid surface, it has been possible, with the aid of the theory of absolute reaction rates, to derive for the rates of adsorption and desorption of gases simple equations which can be tested experimentally. Different results are obtained according as the adsorbed gas forms an immobile or a mobile layer on the surface: in the latter circumstance, which does not appear to be common, the rate of adsorption would be given by the Hertz‐Knudsen equation, provided there were no activation energy for adsorption. Combination of the expressions for the rates of adsorption and desorption gives an adsorption isotherm of the same form as that originally derived by Langmuir. Various cases of adsorption accompanied by dissociation of the adsorbed molecule are considered and the appropriate rate equations are deduced. If the molecule undergoes dissociation in the course of adsorption and the atoms remain on neighboring sites, these equations and the isotherm are the same as if there had been no dissociation, but if the molecule is adsorbed as such and then dissociation occurs as the result of an atom jumping from one adsorption center to another, the rate equations and isotherm differ from those obtained previously. The isotherm involves p☒ instead of p, but the form of the rate equation depends on whether the adsorption of the molecule or the jump of the atom is the slow stage. The possibility of interaction between adsorbed atoms or molecules in an immobile film on the surface is considered, and a modified isotherm which makes allowance for such interaction is derived. Equations have also been deduced for adsorption on covered surfaces.