Abstract: The present invention is a method for separating a gaseous mixture to produce an enriched gas having a predetermined purity which comprises the steps of (a) passing the gaseous mixture into a first adsorption bed at an adsorption pressure in the range from about 15 psig to about 500 psig, (b) withdrawing enriched gas from the first adsorption bed and passing the enriched gas into a product reservoir, (c) when the purity of the enriched gas from the first adsorption bed drops below the predetermined purity thereby becoming lean gas, passing the lean gas from the first adsorption bed into a second adsorption bed to substantially equalize the pressure of the bed, (d) stopping the flow of lean gas from the first adsorption bed and evacuating the first adsorption bed while purging the first adsorption bed by passing enriched gas from the product reservior into the first adsorption bed, (e) further pressurizing the second adsorption bed by passing enriched gas from the product reservoir into the second adsorption b