The atmospheric trend of methane isotopic ratios since the mid-20th century has been reconstructed from Antarctic firn air. High volume air samples were extracted at several depth levels at two sites in East Antarctica. Methane concentration and its 13C/12C and D/H ratios were determined by gas chromatography, mass spectrometry, and infrared spectroscopy. A firn air transport model was applied to reconstruct past atmospheric trends in methane and its isotopic composition. By subsequent application of an atmospheric model, changes in methane sources and OH sink compatible with the past atmospheric trends are explored. In step with increasing methane mixing ratios, δ13C increased by ∼1.7‰ over the last 50 years. These changes mainly reflect a shift in relative source strength toward the heavier anthropogenic methane source, such as biomass burning and methane of nonbiological origin. The δD (CH4) showed a period of decline between the 1950s and 1975, followed by a gradual increase of 0.55‰/yr, also toward the heavier anthropogenic source. Dependent on possible changes in the OH sink, to which δD of methane is very sensitive, the inferred isotopic trends of δ13C and δD over the last 50 years constrain the relationship between natural and anthropogenic sources over the last century. The observed δD minimum around 1975 suggests that the slowing down in the methane source growth took place during this period. Copyright 2001 by the American Geophysical Union.