Separation mechanisms in the adsorption and size-exclusion chromatography (SEC) modes are equivalent in the sense that both processes can be described in terms of a mean fraction of the analyte in mobile phase (MP) and the number of interactions of molecules of the analyte in MP and in pores of/on the solid phase (SP). This is derived by comparing a recent theory of SEC with the theory of Giddings and Eyring for the adsorption chromatography. The elution volume, i.e., the maximum of the elution curve of a species uniform in molecular weight and composition (i.e., the maximum of the spreading function) is the excluded volume divided by the mean fraction of the analyte in mobile phase. Relations between the probabilities of adsorption and desorption of the molecule and the fractions of the analyte in MP and SP were derived. This makes it possible to describe the SEC and adsorption separation mechanisms in the framework of a single unified theory, which is in accord with the Flodin model of separation.