The effect of an addition of 21% oxygen to the N2 carrier gas used for infrared gas analyses of CO2 has been studied at 760 mm Hg and 500 mm Hg pressures by using two widely used infrared gas analyzers. For the Applied Physics unit the results of CO2 analyses using a (21% O2+79% N2) gas mixture as a carrier gas are about 1.7% greater than those obtained by using pure N2 gas as a carrier. On the other hand, for the Unor unit the results of CO2 analyses using air as a carrier gas re about 0.6% smaller than those obtained by using pure N2. A change of pressure from 760 mm Hg to 500 mm Hg appears to cause a change of about 0.2% in the results of CO2 analyses. Since the worldwide investigations of CO2 in the atmosphere and the surface ocean water have been and are being conducted by using either pure N2 or air as a carrier in the calibration gas mixtures, the observed effect of oxygen on the CO2 analysis may be used to correct the data obtained by using N2 as a carrier gas to the CO2 concentrations in air.