Recent advances in the commercial separation of gases using membranes have renewed interest in the possibility of applying this technology to the recovery of methane (ch4) from mining operations. This Bureau of Mines report briefly reviews the history of the development of membranes for gas separation, the theory of how they work, and their application to the separation of methane from air and associated problems. However, methane-air mixtures are difficult to separate with membranes because of pertinent gas couples, o2-n2, o2-ch4, and n2-ch4, have poor separation characteristics, as indicated by their separation factors of about 3 or less. Even if these separation factors were substantially higher, there is doubt that methane could be recovered economically from the low concentrations in mine ventilation exhaust (2 vol pct or less). The exhaust pressures are not sufficient for adequate separation. The power cost of compressing these mixtures would far exceed the value of the methane recovered. New discoveries could make separation of gob hole methane-air mixtures practical. These mixtures have much high concentrations of methane (from 30 to 100 vol pct); however, for safety reasons, treatment would be limited to gob gas with 60 vol pct ch4 or more.