We consider the thermodynamics of adsorption of gases in porous solids from both the perspective of absolute properties which appear naturally in theoretical studies and that of excess properties which are measured in experiments. Our thermodynamic description starts by treating the gas (or gas mixture) plus porous solid system as a mixture to which we can apply solution thermodynamics. We show that equations for the absolute thermodynamic properties for adsorption in rigid porous materials do not require an explicit reference to the pressure of the fluid confined in the porous material. We discuss how to relate absolute properties to excess properties by using an estimate of the helium void volume. We illustrate the thermodynamic formalism with calculations for a simple thermodynamic model in which the Langmuir equation is used to describe the absolute adsorption isotherm and the ideal gas equation of state is used for the bulk properties. The simplified model explains the apparently anomalous behavior of the thermodynamic functions for adsorption at high pressure up to 1000 bar.