The radiocarbon (14C) and stable isotopic (13C and D) compositions of methane and carbon dioxide from alasses (typical landforms in permafrost terrain, consisting of lakes and wetlands) were measured around Yakutsk, eastern Siberia, where few isotope studies have been done. The carbon isotopic compositions of methane and carbon dioxide were used to study the pathways of methane formation in alasses. The mean value of methane and of carbon dioxide in each alass ranged from −63.9 to −58.2‰ and from −16.7 to −0.6‰, respectively. In small alasses, where the supply of fresh organic matter from surrounding wetland ecosystems is large, methane was produced almost equally from acetate fermentation and CO2 reduction pathways. In larger alasses the CO2 reduction pathway slightly dominates over acetate fermentation, owing to a smaller supply of labile organic matter. The 13C enrichment in CO2 in large lakes indicates depletion of methane precursors. Gas-bubble methane is depleted in deuterium (the mean δD value from lakes is −360 ± 14‰ and from shores is −380 ± 20‰), which reflects the deuterium-depleted environmental water (from −136 to −117‰) of alasses. Lake methane relatively enriched in D is interpreted to be the result of depletion in the hydrogen pool in lake sediments. Methane in shallow lakes is somewhat enriched in 14C relative to modern biogenic methane and is produced from fresh, recently fixed organic matter. The 14C content of methane from deeper lakes is 10–20% less than that of shallow lake methane, indicating a greater contribution from older methane derived from deeper parts of the sediment. The mean 13C, D, and 14C of methane from the alasses in general are estimated to be −61.1 ± 4.4‰, −363 ± 20‰, and 104 ± 6 pMC, respectively. This corresponds to the reported mean isotopic composition of methane from wetlands for δ13C, but shows lower value for δD and 14C content.