The report introduces a computer program called CO2SYS, developed by Ernie Lewis and Doug Wallace from Brookhaven National Laboratory, to calculate parameters of the carbon dioxide (CO₂) system in seawater and freshwater. The program can handle four measurable parameters: total alkalinity (TA), total inorganic CO₂ (TCO₂), pH, and either fugacity (fCO₂) or partial pressure (pCO₂) of CO₂. It allows users to select from various formulations of dissociation constants, pH scales, and other parameters. The program is designed for both single-input and batch-input modes, with options for different constant choices, pH scales, and the use of fCO₂ or pCO₂. The report details the input and output requirements, the calculation methods, and the significance of different choices of constants and parameters. It also provides on-screen information and references for further reading. The program aims to facilitate the assessment of CO₂ system calculations and is available for use under specific conditions and with certain limitations.The report introduces a computer program called CO2SYS, developed by Ernie Lewis and Doug Wallace from Brookhaven National Laboratory, to calculate parameters of the carbon dioxide (CO₂) system in seawater and freshwater. The program can handle four measurable parameters: total alkalinity (TA), total inorganic CO₂ (TCO₂), pH, and either fugacity (fCO₂) or partial pressure (pCO₂) of CO₂. It allows users to select from various formulations of dissociation constants, pH scales, and other parameters. The program is designed for both single-input and batch-input modes, with options for different constant choices, pH scales, and the use of fCO₂ or pCO₂. The report details the input and output requirements, the calculation methods, and the significance of different choices of constants and parameters. It also provides on-screen information and references for further reading. The program aims to facilitate the assessment of CO₂ system calculations and is available for use under specific conditions and with certain limitations.