The magnitude of the oceanic source of carbon monoxide (CO) to the atmosphere depends on the concentration of CO in ocean surface waters. To ascertain the relative importance of the processes controlling this concentration, depth profiles of CO concentrations in the oceanic mixed layer and upper thermocline were made at two time series stations in the tropical Pacific Ocean during 1993. The two stations were chosen to contrast an oligotrophic region of low CO water concentrations (RITS93) with a biologically productive region of high seawater CO concentrations (RITS94). The column burden of CO in the upper 75 m of the water column averaged 53 &mgr;mole m-2 at the RITS93 station and 240 &mgr;mole m-2 at the RITS94 station. From the observed diurnal cycle of the CO column burden, production and loss rates at each site were estimated. The factor of 4.5 difference in the column burden is primarily due to a smaller in situ oxidation rate constant at the RITS94 site. ¿ American Geophysical Union 1996