The CO2SYS program is designed to calculate parameters of the carbon dioxide system in seawater and freshwater. It uses two of four measurable parameters (total alkalinity, total inorganic CO2, pH, and either fugacity or partial pressure of CO2) to calculate the other two parameters at specified input and output conditions. The program allows users to select from various pH scales and dissociation constants. It can be run in single-input or batch-input modes, and includes on-screen information and documentation. The program is written in compiled Microsoft QuickBASIC and runs on DOS-based systems. It includes typographical error lists and is supported by the U.S. Department of Energy. The program is intended to help resolve confusion in the calculation of carbonate system parameters by providing a consistent set of choices for constants, pH scales, and other parameters. The program has been tested and validated against various sources, and its accuracy is based on the best available data. The program includes features such as sensitivity analysis, calculations of alkalinity, and saturation states of calcite and aragonite. It also accounts for pressure effects and differences in pH scales. The program is available for use by researchers and is intended to facilitate the assessment of carbonate system calculations.The CO2SYS program is designed to calculate parameters of the carbon dioxide system in seawater and freshwater. It uses two of four measurable parameters (total alkalinity, total inorganic CO2, pH, and either fugacity or partial pressure of CO2) to calculate the other two parameters at specified input and output conditions. The program allows users to select from various pH scales and dissociation constants. It can be run in single-input or batch-input modes, and includes on-screen information and documentation. The program is written in compiled Microsoft QuickBASIC and runs on DOS-based systems. It includes typographical error lists and is supported by the U.S. Department of Energy. The program is intended to help resolve confusion in the calculation of carbonate system parameters by providing a consistent set of choices for constants, pH scales, and other parameters. The program has been tested and validated against various sources, and its accuracy is based on the best available data. The program includes features such as sensitivity analysis, calculations of alkalinity, and saturation states of calcite and aragonite. It also accounts for pressure effects and differences in pH scales. The program is available for use by researchers and is intended to facilitate the assessment of carbonate system calculations.