Multicomponent adsorption equilibria are predicted from theories based on experimental data for single-gas adsorption isotherms. Theories of multicomponent adsorption differ in the details of their results but should agree in special cases such as low surface coverage and ideal solution behavior. This paper is a summary of rules, limits, and consistency requirements that apply to adsorption of single gases and their mixtures. These rules and limits provide a basis for comparing theories with each other and with experimental data.