Characterizing the chemistry of carbon dioxide and its various inorganic forms in seawater is essential to understanding the role played by the ocean in the sea-air exchange of carbon dioxide and the regulation of global climate. To achieve global coverage of carbon dioxide measurements in the sea, scientists and laboratories around the world must cooperate and share data with one another. Nevertheless, this would be a meaningless exercise without some assurance that each laboratory is capable of accurate and precise measurement of seawater carbon dioxide. In this project, the principal investigator proposes to continue his program of producing, certifying, and distributing to the international community a standard reference material for carbon dioxide in seawater. The parameters of the seawater CO2 system that will be certified include total dissolved inorganic carbon (TIC), total alkalinity (TA), hydrogen ion concentration (pH), and the carbon isotopic composition. The objective is to assure intercomparability of oceanic CO2 measurements around the world.