It is now well accepted that the increase of atmospheric greenhouse effect gases, particularly carbon dioxide, has an important impact on our climatic system, even if the true effect of the development of human industrial activities is not totally assessed. It is then necessary to have a good overall knowledge of the carbon dioxide cycle, and more particularly the fluxes exchanged the three CO2 tanks: atmosphere, ocean, and continental biosphere. Since the ocean is the biggest carbon tank (60 times the atmosphere tank), it constitutes an important potenetial well for the atmospheric CO2. However, the data available show an uncertainty of ca. 30% on the estimated flux. It is then important to assess the quantity of CO2 at the surface of the ocean, and the corresponding atmosphere/ocean flux. The possible future atmospheric CO2 concentration corresponding to different industrial emission scenarii can then be assessed. The method measurement of the CO2 at the sea surface use spectro- photometry of a sensitive dye, housed in a tank which has walls immersed in the sea water. The spectro-photometer especially designed, patented, and build-up by IFP has been developed with special emphasis on high accuracy, rugged construction, ease of operation without maintenance, and calibration over a period of time of over one year. The leading idea was to have as few internal moving parts as possible by the use of fiber optics, and internal calibration by the use of internal reference channels. Ancillary data are also measured. Data is transmitted to the processing center via ARGOS. The equipment described is part of the EUREKA CARIOCA project (EU 819), supported by the French Ministere de l'Enseignement Superieur et de la Recherche (93 W 0011) and UK's DTI. The partners are: LODYC, LGE, INSU, SERPE-IESM (France), and Chelsea Instruments (UK).