A methodology for the collection of large (1000 L) air samples for isotopic analysis of atmospheric carbon monoxide is presented. A low-background, high-pressure, high-flow sampling system with a residual background of less than 2 ppbv CO has been built and employed for collection of samples both from the ground and from an aircraft platform. The time required for obtaining a 1000-L sample pressurized to 600 psi was approximately 30 min on the ground, and 75 min at 8-km altitude. Following collection, samples were analyzed for CO concentration, 13C, 18O, and 14C using cryogenic vacuum extraction followed by stable isotope ratio mass spectrometry and accelerator mass spectrometry. Standard deviation for δ13C is 0.2 per mil, for δ18O 0.8 per mil, and for 14CO 0.5 molecules per cubic centimeter (STP). Abstract A methodology for the collection of large (1000 L) air samples for isotopic analysis of atmospheric carbon monoxide is presented. A low-background, high-pressure, high-flow sampling system with a residual background of less than 2 ppbv CO has been built and employed for collection of samples both from the ground and from an aircraft platform. The time required for obtaining a 1000-L sample pressurized to 600 psi was approximately 30 min on the ground, and 75 min at 8-km altitude. Following collection, samples were analyzed for CO concentration, 13C, 18O, and 14C using cryogenic vacuum extraction followed by stable isotope ratio mass spectrometry and accelerator mass spectrometry. Standard deviation for δ13C is 0.2 per mil, for δ18O 0.8 per mil, and for 14CO 0.5 molecules per cubic centimeter (STP).