We report measurements of the vertical distribution of the carbon dioxide mixing ratio in the lower and middle stratosphere. They were performed during a seroes of balloon flights over midlatitudes (44¿N) during different seasons and over high latitudes (68 ¿N) during winter in the time period from 1982 until 1990. CO2 was gaschromatographically determined in air samples collected by means of cryogenic whole-air samplers. The annual increase of the CO2 content of the mid-statospheric air is comparable to the temporal CO2 trend in the troposphere. However, the stratospheric concentrations are lagging the tropospheric ones by a mean delay time of 5.6¿1.1 years. This delay time may be interpreted as the 'age' of stratospheric air masses. It is determined by the efficiency of the transport processes that mix tropospheric air into the stratosphere. Our CO2 observations during the last decade suggest that these processes have varied systematically with a time period of 3 to 5 years. A rather low 'age' of 2.3¿0.3 years was derived from the most recent observations over midlatitudes in summer 1989 as well as in the Arctic region during the CHEOPS III Champaign in winter 1989/90. ¿American Geophysical Union 1991