The concept of single-layer cluster adsorption and a new scientific understanding of single-layer adsorption by porous and non-porous sorbents are proposed. The model expands the understanding of the physical adsorption, where sorbate-sorbate bonds make a significant contribution to the adsorption energy. The fundamental difference between proposed adsorption picture and the traditional one is the representation of one sorption center as the center of fixation of several molecules for porous systems. An analytical expression for adsorption isotherm have been obtained from equilibrium coefficients expressions of the adsorption processes, expressed via surface coverage fraction by adsorbed molecules. The developed model supplements the adsorption theory. A geometric method is described for the parameters determining of the simplified model. The model is applied to the interpretation of CO2 adsorption isotherms by metal-organic frameworks MOF-177, IRMOF-1, IRMOF-6 and IRMOF-11.