Carbon fractionation (Δ 13 C) is well documented for various plants functional types. Yet, specific studies on Δ C on mangroves are particularly rare although they have a key role in coastal carbon (C) cycling. In this study, we investigated the 13 C exchanges between leaves and the atmosphere and between the main plant's organs in two common mangroves species, Avicennia marina and Rhizophora stylosa subjected to two different CO2 concentrations. Two-years-old saplings were grown in mesocosms during one year under 400 ppm and 800 ppm of CO2. At the end of the experiment, the isotopic value of the night-respired CO2 was measured on six individuals for each species and CO2 treatment. Then, 60 saplings were harvested to measure the organs δ 13 C values, and, finally, carbon fractionation (Δ 13 C) during photosynthesis, post-photosynthesis and apparent Δ 13 C during night respiration were calculated. Results indicated that elevated CO2 reduced Δ 13 C during photosynthesis by 13 % and during night respiration by 20 %. Alongside, within-plant Δ 13 C was twice higher in the saplings grown under elevated CO2 concentrations. These results showed that ongoing and future increases in atmospheric CO2 concentrations have the potential to modify the δ 13 C values of mangrove trees. These results could have important implications in Blue Carbon sciences, and particularly in the comprehension of future carbon cycling in coastal wetlands, mangroves being an essential link in terrestrial and marine food webs along tropical and subtropical coastlines.