An inventory of natural gas losses from the former Soviet Union's gas industry has been constructed from published Russian-language sources. The results imply that in the late 1980s/early 1990s annual losses from Russia were in the range 35–59×109 cubic meters (24–40 Tg of CH4): estimates based on what are thought to be the more reliable sources place annual losses in the range 37–52×109 cubic meters (25–35 Tg of CH4). Of this amount, one half to two thirds of the emissions may have been from the extremely long and ageing gas pipeline system. Extrapolation of the estimates for Russian losses to the whole territory of the former Soviet Union suggests a probable total annual emission level from the whole ex-Soviet gas industry in the range 47–67×109 cubic meters of natural gas or 31–45 Tg of CH4 in these years. The envelope of minimum and maximum estimates for emissions from the former Soviet Union ranges from 29 to 50 Tg of methane. The limited availability of systematic and accurate published information on the emissions introduces significant uncertainty into the estimate. In an attempt to constrain emissions better, estimates of losses from specific causes were made using two or more independent approaches, where possible. A reasonable agreement between estimates was achieved in those cases. Our results imply that substantial reductions in emissions could be achieved by investment to reduce losses. Because of the high global warming potential and short lifetime of methane compared to carbon dioxide, reducing the large losses from the FSU may be among the most cost-effective short-term approaches available to reduce global anthropogenic greenhouse warming.