Introduction In recent years@ several investigations have revealed that carbon monoxide may have a more active and complicated role in nature than had generally been known. Swinnerton et al. [1] and Seller and Junge [2] have discovered that the oceans may be a significant source of CO. Seller and Junge [3] have shown that the tropopause is a sink for CO and Inman et al. [4] found that soils may also be a significant sink. Junge et al. [51 have discussed the global atmospheric budget of CO@ assuming only anthropogenic and ocean water sources. The abundance of the stable isotopes of CO might be a way of distinguishing CO from different origins and thus identifying the origins of atmospheric CO. Both carbon and oxygen have quite variable isotopic compositions in nature [6-8]. At almost every stage of any natural process involving these elements@ there are isotopic fractionation effects. The combination of isotopic variations for two elements having different cycles might be of special diagnostic value. A program of measuring the isotopic composition of CO in the atmosphere and from natural sources has been going on at Argonne since 1969. The atmospheric studies include two separate problems: (1) the determination of the global-average isotopic composition of CO emitted by internal combustion engines@ and (2) the determination of the isotopic pattern of CO in the least polluted atmosphere (mostly in Illinois) at different seasons and@ where possible@ different latitudes and times of day.