Human activities have caused the CO2 concentration to increase steadily in the atmosphere of the Globe, the resulting 'greenhouse' effect influencing potentially the future climate on global and regional scales alike. Advances have been registered in research into the meteorological and hydrological impacts of the greenhouse effect, whereas studies on water quality have barely started. Water quality can be influenced on many different ways. Higher water temperatures may result primarily in accelerated biological metabolic processes, but also changing the stratification, sediment nutrient release and ice conditions of lakes. The changes in the amount and distribution of precipitation will modify runoff conditions: in the Carpathian Basin a reduction is expected. Due to alterations in runoff, nutrient loads of non-point source origin and residence time of lakes will be modified, too. All the above factors may significantly influence eutrophication of standing waters. Higher CO2 concentrations in the atmosphere may have also direct impacts on the chemical equilibrium processes of the inorganic carbon system of waters. It may entail increases in the salt content, especially for waters of typically Ca(HCO3)2 character, which is the case for most of the shallow water bodies of Hungary, including Lake Balaton. The latter potential impact on the chemical composition of hard waters have been demonstrated on the basis of classical equilibrium equations, by assuming a twofold increase in the CO2 level.