The main aspects of an absolute method for measurement of the mixing ratio of atmospheric carbon monoxide (CO) are presented. The method is based on cryogenic extraction of CO from air after oxidation to CO2 followed by accurate volumetric determination. Gravimetric measurement is used to determine the quantity of sample air processed. In routine operation the overall error can be kept below 1%. Furthermore, the results of a laboratory intercomparison are analyzed. It is shown how offsets in the commonly applied analytical methods can occur and how these can seriously affect results obtained at the low concentration end (<100 nmole/mole). ¿ 2001 American Geophysical Union