A two-step technique, in which the N2O/CO2 ratio is measured by ultrasonic phase-shift gas chromatography and the dry air CO2 concentration is measured by nondispersive infrared analysis, has been developed for the determination of the mole fraction of nitrous oxide in dry air. The N2O cemcentration is given by the product of these independent measurements and has a presision (¿1 standard deviation) ranging between 0.3 and 0.5%. The abscence of systematic errors has been varified by existensive standard intercomparisons and byindependent cross checks of the sample extraction procedures. The results of extensive measurements by this technique, reported in a companion paper, fix the mean tropospheric dry air mole fraction of nitrous oxide in the northern hemisphere as of January 1, 1978 at 300.2¿0.6 parts per billion, including systematic uncertainties.