In order to study the vertical distribution of the different chemical species in the dissolved CO2 system of water off the Chilean coast, seawater samples were taken at two oceanographic stations near Coquimbo, down to 2,800 m depth. Precise measurements of dissolved oxygen, pH and total alkalinity (A(T)) at 25 degrees C were made. Knowing the in situ salinity and temperature, it is possible to estimate the apparent dissociation constants required to describe the CO2 system equilibrium, in a complex matrix such as seawater, in which high ionic strength and large temperature differences exist.