The determination of CO2 saturation by two different methods is described for freshwater bodies of differing trophic states, hydrology and chemistry in the North-East of Germany. Direct measurements were carried out by gas chromatography and values were calculated from the dissolved inorganic carbon concentration and pH. These results were in good agreement. In both cases, the CO2 saturation index was calculated from the carbon dioxide/carbonate/hydrogencarbonate equilibrium in water. An overestimation of CO2 saturation index will be caused at pH > 7 by neglecting the calcium forming ion pairs with HCO3– and CO32–. The CO2 saturation patterns exhibited seasonal changes in all the lakes investigated which were variable within each trophic group. A direct relationship was found between the increasing trophic state and undersaturation of CO2 during the periods of high primary production. Thus, with respect to the atmosphere, these freshwater bodies act mostly as sources of CO2. Periods of ice covering and clear-water phases are characterized by high CO2 supersaturation and therefore the surface waters investigated are CO2 sources for short periods only.