The upgrading of low-quality natural gas by membrane separation processes utilizing two different types of polymer membranes was investigated by process design. optimization. and economic assessment studies. The membranes considered for these processes exhibit high CO2/CH4 and H2S/CH4 selectivities. respectively. Low-quality natural gas was simulated by ternary CH4/CO2/H2S mixtures containing up to 40 mol% CO, and 10 mol% H2S. The membrane processes were designed to reduce the concentrations of CO, and H,S in the natural gas to US pipeline specifications (less than or equal to2 mol% CO, and less than or equal to4 ppm H2S). This study has identified the membrane process configurations (single stage or two stages connected in series) and types of membranes (M-selective, H2S-selective, or both) that would yield the most economical processes for the upgrading of natural gas containing different ranges Of CO2 and H2S concentrations. The upgrading costs of process configurations without recycle streams are dominated by the cost of CH4 lost in the permeate stream. The effects of recycle streams, variations in feed flow rate and pressure, membrane module cost, and wellhead cost of natural gas on the upgrading costs will be reported elsewhere.