Accurate measurements of carbon monoxide (CO) in humid air have been made usingthe cavity ring-down spectroscopy (CRDS) technique. The measurements of CO molefractions are determined from the strength of its spectral absorption in the near infraredregion (∼ 1.57 µm) after removing interferences from adjacent carbon dioxide (CO25 )and water vapor (H2O) absorption lines. Water correction functions that account forthe dilution and pressure-broadening eﬀects as well as absorption line interferencesfrom adjacent CO2 and H2O lines have been derived for CO2 mole fractions between360–390 ppm. The line interference corrections are independent of CO mole fractions.10 The dependence of the line interference correction on CO2 abundance is estimated tobe approximately −0.3 ppb/100 ppm CO2for dry mole fractions of CO. Comparisons ofwater correction functions from diﬀerent analyzers of the same type show signiﬁcantdiﬀerences, making it necessary to perform instrument-speciﬁc water tests for eachindividual analyzer. The CRDS analyzer was ﬂown on an aircraft in Alaska from April to15 November in 2011, and the accuracy of the CO measurements by the CRDS analyzerhas been validated against discrete NOAA/ESRL ﬂask sample measurements madeon board the same aircraft, with a mean diﬀerence between integrated in situ and ﬂaskmeasurements of −0.6 ppb and a standard deviation of 2.8 ppb. Preliminary testingof CRDS instrumentation that employs new spectroscopic analysis (available since20 the beginning of 2012) indicates a smaller water vapor dependence than the modelsdiscussed here, but more work is necessary to fully validate the performance. TheCRDS technique provides an accurate and low-maintenance method of monitoring theatmospheric dry mole fractions of CO in humid air streams.