Abstract: The Advanced Global Atmospheric Gases Experiment (AGAGE) program and the National Oceanic and Atmospheric Administration (NOAA) Global Monitoring Laboratory (GML) maintain some of the longest measurement records of the atmospheric abundances of ozone-depleting substances (ODSs) and their substitutes, many of which are strong greenhouse gases (GHGs). Several records extend back more than 40 years, and fittingly, Walt Komhyr was involved in the early halocarbon measurements at NOAA in the mid-1970s. For the past 20+ years, comparisons of ozone-depleting substances and non-CO₂ greenhouse gas measurements have been carried out and reviewed by project participants for more than 45 species twice yearly. AGAGE and NOAA carry out measurements at six common sites: Cape Grim, Australia; Cape Matatula, American Samoa; Ragged Point, Barbados; Trinidad Head, USA; Mace Head, Ireland; and Zeppelin, Norway. Cylinders containing compressed air have also been exchanged for measurement in our central laboratories. Such ongoing comparisons serve to establish calibration scale relationships, as well as any offsets that may occur during sampling and/or measurement, that can be used to convert NOAA data to AGAGE data, or vice versa, to combine the records for joint modeling studies. These ongoing comparisons aid the quality control for both networks, and alert scientists to potential measurement or calibration issues, including their possible temporal and/or spatial variations. Our goal is that calibration relationships be calculated in a consistent manner and be made available for general use. Examples of some long-term records and how they might be interpreted, the calibration relationships that arise from co-located measurements, and the methods used to derive those relationships will be discussed. We intend to include and regularly update these relationships on the AGAGE and NOAA/GML websites.