A procedure for the determination of the 13C/12C ratio and the 14C abundance in atmospheric methane is presented. The method is based on the collection of air samples in stainless steel tanks at a pressure of 7 bar. The air is processed in the laboratory by cryogenic removal of condensibles, followed by oxidation of the methane content after which the resulting CO2 is collected. Also CO is removed prior to oxidation. The 13C/12C ratio is determined on the CO2 sample by stable isotope ratio mass spectrometry. The 14C content is determined by means of accelerator mass spectrometry. The overall precision of the technique is 0.1‰ for δ13C and 1.5 pMC for 14C. The method has been used to determine the carbon isotopic composition of methane in air collected at Baring Head, New Zealand, and also in air collected on aircraft flights between New Zealand and Antarctica. No gradient in the carbon isotopic composition between Baring Head and South Pole station was detected. The annual mean δ13C value at Baring Head was −47.13±0.20‰ for 1989 which includes seasonal effects probably due to OH variations and local meteorology. The annual mean 14C value at Baring Head in 1989 was 118.3 percent modern (pMC).