An assembly of interacting particles which may adsorb onto a substrate is considered. Using the Ornstein–Zernike equation, some general expressions are derived relating the adsorption excess to the direct correlation function between the adsorbate and adsorbent, and the compressibility of the absorbate.Invoking a mean spherical model (MSM) approximation, which identifies the direct correlation function between adsorbate and adsorbent with an appropriate Mayer function, two cases are then considered in detail. These are the cases of long and short range Mayer functions. The result obtained for the former case agrees with a result derived by Findenegg using a different approach. By comparing with experiment, it is argued that the latter case (short range Mayer function) is more appropriate to describe simple gas adsorption.