The mass transfer at the gas-separation step of pressure swing adsorption in hybrid membrane-sorption system, which is composed of series-connected pressure swing adsorption stage and membrane stage, has been studied. In a hybrid system, the adsorber filling is performed by retentate flow from the membrane stage. Based on a comparison of the mass transfer in a single stage of pressure swing adsorption and hybrid system, the new principles of the process have been determined, namely, identity on the parameters of external conditions of hybrid system with the adsorber filling by retentate flow and single stage of pressure swing adsorption with combined adsorber filling by product and feed flows, which has the same value of product flow.