[1] We present variations of CO 2 in the tropopause region obtained by frequent in situ measurements aboard commercial aircraft. The data were obtained from a total of 373 flights between Japan and Europe during the period November 2005 to September 2007. The local phase and amplitude of the CO 2 seasonal cycle varied with distance from the tropopause. In the upper troposphere and in the region just above the dynamical tropopause, a strong seasonal cycle with a springtime maximum and a relatively sharp minimum in July was observed. In the region bounded by potential temperatures 10 K to 30 K above the extratropical tropopause, no significant seasonal cycles were found. In the region greater than 30 K from the tropopause (i.e., at higher altitudes), sharp CO 2 increases in summer followed by gradual decreases were found, resulting in a slightly increasing seasonal cycle amplitude with distance from the tropopause. The observed CO 2 distributions also showed that CO 2 isopleths followed the tropopause during the winter and spring, whereas in the summer they tracked potential temperature surfaces crossing the tropopause. The observed seasonal variation in CO 2 suggests that the lowermost stratospheric region is influenced by a combination of (1) fast meridional transport of high CO 2 from the tropical troposphere in the summer, (2) active subsidence of low CO 2 from higher altitudes in the spring, and (3) relatively weak vertical mixing near the tropopause.