Summary:Measurements have been made on air collected in flasks at sites in the CSIRO GASLAB worldwide network. These sites include: Alert, Northwest Territories, Canada; Shetland Islands, Scotland; Estevan Point, British Columbia, Canada; Mauna Loa, Hawaii; Cape Ferguson, Australia; Cape Grim, Australia (Tasmania); Macquarie Island, Australia; Mawson, Antarctica; and the South Pole station, Antarctica. Flasks are filled with air at the CSIRO GASLAB sites and returned to the CSIRO GASLAB for analysis; typical sample storage times for flasks collected at the CSIRO GASLAB sites are from a few weeks to a few months. No significant effect on the stable carbon isotopic composition (13C) has been detected as a consequence of the sample storage time. For stable isotope analysis, CO2 was extracted from the air in the flasks using an automated cryogenic trapping system. Prior to stable isotopic analysis, the concentrations of CO2, and N2O were measured in all samples using gas chromatography (Francey et al., 1996). The CO2 and N2O concentrations were used to perform ion corrections on the stable isotope measurements required as a consequence of extracting the isobaric N2O from air with the CO2. The stable isotopic composition of the extracted CO2 was measured by dual inlet stable isotope ratio mass spectrometry (ThermoFinnigan MAT252). The normal mass spectrometric measurement of delta 13C in atmospheric CO2 requires the simultaneous measurements of ion currents due to three mass/charge (m/z) ratios: 44, 45, and 46. Information contained in the m/z 46 ratio is used to quantify and remove the 12C 16O 17O isobaric interference to the measurement of 13C 16O2. After isobaric correction (17O and N2O), the delta 13C was adjusted onto the VPDB-CO2 scale using bracketing measurements of air with known CO2 stable-isotope composition, maintained in high-pressure cylinders. All air standards used in GASLAB since 1990 remain in use to monitor long-term calibration and to ensure continuity of calibration through new air standards. The link to VPDB-CO2 has been monitored by comparisons between a suite of sub-samples of a single high-purity gas calibrated against NBS-19 carbonate (Allison and Francey, 1995) and a number of other high-purity CO2 gas standards (GS-19, GS-20, OZTECH-3, OZTECH-30, OZTECH-40, NIST RMs 8562, 8563 and 8564). Further checks on the calibration of CO2 measurements extracted from air samples onto the VPDB-CO2 scale are maintained through regular analysis of high-pressure air standards, such as the IAEA CLASSIC suite (Allison et al., 2002, 2003), and flask-air sharing comparisons (e.g., Masarie et al., 2001) maintained between CSIRO and various international laboratories. More detailed calibration information is available in Langenfelds et al. (2001). The data presented here are monthly means, calculated as the mean of the daily values obtained from a smooth curve fit to the flask data using techniques of Thoning et al. (1989). For an interpretation of the trends derived from the time series, please visit the data set descriptions in Trends Online at []. References: Allison, C. E., and R.J. Francey, 1995. High precision stable isotope measurements of atmospheric trace gases, In Reference and intercomparison materials for stable isotopes of light elements: proceedings of a consultants meeting, Vienna (IAEA-TECDOC-825). Vienna, Austria: International Atomic Energy Agency. p. 131-153. Allison, C.E., R.J. Francey, and L.P. Steele, 2002. The International Atomic Energy Agency circulation of laboratory air standards for stable isotope comparisons: Aims, preparation and preliminary results. In Isotope aided studies of atmospheric carbon dioxide and other greenhouse gases Phase II (IAEA-TECDOC-1269). Vienna, Austria: International Atomic Energy Agency, p. 5-23. Allison, C.E., R.J. Francey, J.W.C. White, B. Vaughn, M. Wahlen, A. Bollenbacher, and T. Nakazawa, 2003. What have we learnt about stable isotope measurements from the IAEA CLASSIC? In Report of the Eleventh WMO/IAEA Meeting of Experts on Carbon Dioxide Concentration and Related Tracer Measurement Techniques, Tokyo, Japan, 25-28 Sep. 2001, WMO/GAW Report No. 148, p. 17-30, Geneva. Francey, R.J., L.P. Steele, R.L. Langenfelds, M.P. Lucarelli, C.E. Allison, D.J. Beardsmore, S.A. Coram, N. Derek, F.R. de Silva, D.M. Etheridge, P.J. Fraser, R.J. Henry, B. Turner, E.D. Welch, D.A. Spencer, and L.N. Cooper, 1996. In Global Atmospheric Sampling Laboratory (GASLAB): supporting and extending the Cape Grim trace gas programs. Baseline Atmospheric Program (Australia) 1993, edited by R.J. Francey, A.L. Dick and N. Derek, pp 8 - 29, Bureau of Meteorology and CSIRO Division of Atmospheric Research, Melbourne, Australia. Langenfelds, R.L., L.P. Steele, C.E. Allison, and R.J. Francey, 2001. CSIRO GASLAB Calibration, 2001. Internal Report, CSIRO Atmospheric Research, Aspendale, Australia. Masarie, K.A., R.L. Langenfelds, C.E. Allison, T.J. Conway, E.J. Dlugokencky, R.J. Francey, P.C. Novelli, L.P. Steele, P.P. Tans, B. Vaughn, and J.W.C. White, 2001. NOAA/CSIRO Flask Air Intercomparison Experiment: A strategy for directly assessing consistency among atmospheric measurements made by independent laboratories, J. Geophys. Res. 106, 20445-20464. Thoning, K.W., P.P. Tans, and W.D. Komhyr, 1989. Atmospheric carbon dioxide at Mauna Loa Observatory, 2, Analysis of the NOAA/GMCC data, 1974 - 1985, J. Geophys. Res. 94, 8549-8565.