We developed and tested a new method to separate CO₂ and CH₄ from bulk gas samples for radiocarbon and stable-carbon analysis that utilizes a zeolite molecular sieve. To validate the technique, tests were performed using a suite of standard gases, composed of CO₂ and CH₄ of distinctly different isotopic composition. We employed the method to investigate the carbon isotopic composition of samples of dissolved CO₂ and CH₄ collected in situ from the near surface to deep layers of an ombrotrophic raised peat bog. Results showed that the age of both the CO₂ and CH₄ components of the dissolved gases increased with depth from ≃0–300 BP at 0.25 m to ≃4000 BP at 4 m. CH₄ was mainly similar or slightly older in age compared to CO₂, with the greatest difference in ages occurring at 1 m depth where CH₄ was older by 430–615 yr. The δ¹³C values of CO₂ increased with depth from −12.4‰ and −8.0‰ at 0.25 m to +6.9‰ and +8.3‰ at 4 m, whereas the δ¹³C of CH₄ stayed in the range −58.4‰ to −70.6‰. The ¹⁴C results from the deepest layers are consistent with a similar source for both gases. ¹⁴C ages for the CO₂ component were younger compared to CH₄, within the shallower depths of the peat bog (≤1 m) and demonstrate the incorporation of acrotelm-derived respired CO₂ into the catotelm.