CO 2 occupies a major place in the biosphere, and its exchange between the atmosphere and ocean plays an important role in global climate control. The ocean is able to absorb large quantities of CO 2, due to its solubility in water; the solubility is increased by the buffering action of the carbonate-bicarbonate system in sea water. In order to study the vertical distribution of the different chemical species in the dissolved CO 2 system of water off the Chilean coast, seawater samples were taken at two ocWanographic stations near Coquimbo, down to 2,800 m depth. Precise measurements of dissolved oxygen, pH and total alkalinity (A T at 25°C were made. Knowing the in situ salinity and temperature, it is possible to estimate the apparent dissociation constants required to describe the CO 2 system equilibrium, in a complex matrix such as seawater, in which high ionic strength and large temperature differences exist.