ABSTRACT In this work, we present CO2, latent heat and sensible heat fluxes measured over the reservoir of the Itaipu Hydroelectric Power Plant (Paraná State, Brazil) during 2013. A tower was installed at a small island in the reservoir, where an Eddy Covariance system, with supplementary equipments and analysers, was deployed. The objective of this work was to determine the magnitude of CO2 fluxes and their variation throughout the year. CO2 flux displayed seasonality: in warm months there was a predominance of negative CO2 daytime fluxes and positive CO2 nighttime fluxes, which we attributed to photosynthesis/respiration in the reservoir; in cold months there was a predominance of negative CO2 fluxes in both periods, with CO2 air concentration apparently imposing the signal of the CO2 fluxes and the strong winds intensifying them. The range of 90% of the CO2 fluxes measured in this work (−102.68 to +151.72 µg m−2s−1 at the 30-min. time scale) is comparable to those observed in natural lakes and reservoirs around the world. On the average, the reservoir acted as a source of CO2, with an overall mean flux of +12.78 µg m−2s−1.