The Baltic Sea is characterized by low inorganic alkalinities and high concentrations of dissolved organic matter (DOM). The contribution of DOM to the marine acid-base system has mostly been ignored in previous studies based on the assumption that coastal waters exhibit the same conditions as oceanic waters. The present thesis investigated concentrations and acid-base properties of total DOM, humic/fulvic acids and carboxylic acids. Conducted field, laboratory and theoretical studies revealed a not negligible contribution of organic compounds to the alkalinity in the Baltic Sea.