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Stable carbon isotope analysis of methane (&delta;¹³C of CH₄) on atmospheric samples is one key method to constrain the current and past atmospheric CH₄ budget. A frequently applied measurement technique is gas chromatography (GC) isotope ratio mass spectrometry (IRMS) coupled to a combustion-preconcentration unit. This report shows that the atmospheric trace gas krypton (Kr) can severely interfere during the mass spectrometric measurement, leading to significant biases in &delta;¹³C of CH₄, if krypton is not sufficiently separated during the analysis. According to our experiments, the krypton interference is likely composed of two individual effects, with the lateral tailing of the doubly charged ⁸⁶Kr peak affecting the neighbouring <i>m/z</i> 44 and partially the <i>m/z</i> 45 Faraday cups. Additionally, a broad signal affecting <i>m/z</i> 45 and especially <i>m/z</i> 46 is assumed to result from scattered ions of singly charged krypton. The introduced bias in the measured isotope ratios is dependent on the chromatographic separation, the krypton-to-CH₄ mixing ratio in the sample, the focusing of the mass spectrometer as well as the detector configuration and can amount to up to several per mil in &delta;¹³C. Apart from technical solutions to avoid this interference, we present correction routines to a posteriori remove the bias.