Carbon isotopes are commonly used to study how CO2 diffuses within photosynthetic plant tissues, information that is difficult to obtain with common gas exchange techniques alone. The standard method used to interpret the observed preference for the lighter carbon isotope in C3 photosynthesis involves the model of Farquhar et al. (1982), which relates carbon isotope discrimination to physical and biochemical processes within the leaf. However, under many conditions the model returns unreasonable results for mesophyll conductance to CO2 diffusion, including values that are negative. Florian will discuss a re-derived carbon isotope discrimination model that uses modified assumptions related to the isotope effect of mitochondrial respiration. This new theory allows for estimating mesophyll conductance accurately, especially under conditions when rates of photosynthesis are low. Florian will further discuss how the changes to the model influence estimates of CO2 diffusion properties within the mesophyll and the implications this has for predictions of plant carbon uptake.