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Isotope records of atmospheric CH₄ can be used to infer changes in the biogeochemistry of CH₄. One factor currently limiting the quantitative interpretation of such changes are uncertainties in the isotope measurements stemming from the lack of a unique isotope reference gas, certified for δ¹³C-CH₄ or δ²H-CH₄. We present a method to produce isotope reference gases for CH₄ in synthetic air that are precisely anchored to the VPDB and VSMOW scales and have δ¹³C-CH₄ values typical for the modern and glacial atmosphere. We quantitatively combusted two pure CH₄ gases from fossil and biogenic sources and determined the δ¹³C and δ²H values of the produced CO₂ and H₂O relative to the VPDB and VSMOW scales within a very small analytical uncertainty of 0.04‰ and 0.7‰, respectively. We found isotope ratios of −39.56‰ and −56.37‰ for δ¹³C and −170.1‰ and −317.4‰ for δ²H in the fossil and biogenic CH₄, respectively. We used both CH₄ types as parental gases from which we mixed two filial CH₄ gases. Their δ¹³C was determined to be −42.21‰ and −47.25‰ representing glacial and present atmospheric δ¹³C-CH₄. The δ²H isotope ratios of the filial CH₄ gases were found to be −193.1‰ and −237.1‰, respectively. Next, we mixed aliquots of the filial CH₄ gases with ultrapure N₂/O₂ (CH₄ ≤ 2 ppb) producing two isotope reference gases of synthetic air with CH₄ mixing ratios near atmospheric values. We show that our method is reproducible and does not introduce isotopic fractionation for δ¹³C within the uncertainties of our detection limit (we cannot conclude this for δ²H because our system is currently not prepared for δ²H-CH₄ measurements in air samples). The general principle of our method can be applied to produce synthetic isotope reference gases targeting δ²H-CH₄ or other gas species.