Abstract : Leaf photosynthetic activity depends on the CO2 availability at Rubisco. This availability arises from a series of limitations to CO2 transfer between ambient air and carboxylation sites in the chloroplasts, resulting in a mole fraction gradient. A large part of this gradient is the consequence of a limitation to CO2 transfer within the mesophyll, starting in the intercellular gaseous phase, and ending in the cellular liquid phase. This limitation within mesophyll is quantified as a conductance to CO2 transfer i.e. the internal conductance(gi). The aim of this work was to study CO2 diffusion within the mesophyll for plants with a C3 metabolism. First, we improve the gi estimation using a method combining measurements of gas exchange and chlorophyll fluorescence. We subsequently analyse the two components determining gi : the limitation to CO2 transfer in the intercellular air spaces network, and the limitation in the liquid phase within cells. Using an original approach for gi estimation in an helium based atmosphere, we show that the main component of gi is the limitation to CO2 transfer in the cellular liquid phase. Finally, we studied the interspecific and phenotypic variability of gi. Our results confirm the existence of an interspecific positive correlation between gi and maximum net assimilation, and show that the putative discrepancy between typically low gi in ligneous species and typically high gi in herbaceous species is not valid. We also show that leaf acclimation to irradiance in walnut go together with gi variations, and we propose a way to scale gi into photosynthetic models.