BACKGROUND Biogas is an alternative renewable energy produced by anaerobic digestion of various organic wastes mainly from agriculture, households and biomass and food industries. Typically, raw biogas consists of 50-70% methane (CH4), 30-50% carbon dioxide (CO2), and small amounts of hydrogen sulfide (H2S) and water vapour. Purifying biogas by removing CO2 and other trace impurities is required in order to achieve higher calorific value, safe operation and to meet fuel standards. As CO2 is the second largest component of biogas, this study focused on its removal from CH4. RESULTS Composite membranes of blended PEG 400/Pebax 1657 polymer and graphene oxide (GO) were developed for CO2/CH4 gas separation. The effects of GO loading and PEG 400 addition on CO2/CH4 separation performance were investigated. Optimal loadings of 0.25 wt% GO and 50 wt% PEG 400 obtained the best membrane performance with 12.4 GPU of CO2 permeance and 14 of CO2/CH4 separation factor. CONCLUSION Incorporating GO to the polymer membrane enhanced CO2/CH4 separation, whereas PEG 400 addition increased CO2 permeance. The advantages from each component led to an enhancement of CO2 permeance and CO2/CH4 separation. (c) 2019 Society of Chemical Industry