Measurements of the isotopic composition of methane in the Antarctic atmosphere have indicated the presence of either 12CD4 or 12CHD3. The concentration is about 1.3¿10-16(v/v). This concentration is at least 500 times higher than expected from the statistical combination of the natural isotopic abundances of H, D, 12C and 13C. There are no known sources of these species large enough to explain these concentrations. It is possible that these species have accumulated in the atmosphere by virtue of having atmosphere residence times that are substantially longer than the residence time of CH4. ¿ American Geophysical Union 1989