Simultaneous 17O/16O and 18O/16O measurements of atmospheric carbon monoxide (CO) and two of its major sources are presented. CO was collected in situ at ground level in La Jolla, CA, Pasadena, CA and White Mountain, CA, altitude 3.8 km. Samples were also taken directly from engine exhaust and during a local wild brushfire. Results indicate that atmospheric carbon monoxide has a significant mass independent component, which increases as carbon monoxide concentrations decrease. The origin of the observed mass independent fractionation is discussed. CO samples from engine exhaust and brushfires are clearly mass dependent, but with a wide variation in 18O values from different engines. ¿ 1998 American Geophysical Union