A high-precision manometric system has been developed for absolute calibrations of CO2-in-air mixture gas. This report describes the principle of the calibration method and evaluates the performance of the manometric apparatus. The test results show that the reproducibility of the manometric system for calibrating CO2-in-air gas mixtures is within ¿0.1 μmol mol-1 in the atmospheric CO2 concentration range of 300 to 400 μmol mol-1. Preliminary measurements indicate that the agreement with the absolute World Meteorological Organization mole fraction scale, widely used for atmospheric CO2 monitoring, is also within ¿0.1 μmol mol-1 in that same range.¿ 1997 American Geophysical Union