William Martin teaches this session of MIT's Marine Chemistry (12.742, Fall 2006), covering the chemistry of inorganic carbon dissolved in seawater. He explains why carbonate chemistry matters for ocean processes, how researchers measure dissolved inorganic carbon and alkalinity, and how these measurements tie into major biogeochemical cycles such as carbon burial and calcium carbonate formation. The lecture works through the equilibrium reactions that govern carbon dioxide, bicarbonate, and carbonate ion concentrations in ocean water, and situates this chemistry within the broader questions of ocean acidity and the ocean's role in the global carbon cycle. Delivered in a standard classroom lecture format with a chalkboard and running commentary, it assumes some background in chemistry and is aimed at graduate or advanced undergraduate students studying oceanography or marine chemistry.