Abstract Molecular diffusion of gases in heavy oils is one of the most important physical parameter governing cold production processes such as solution gas drive or Vapex. Indeed, the way bubbles are able to grow by diffusion will have a direct impact on gas mobilization and consequently on oil recovery. Furthermore, the importance of this parameter is emphasized by the very low gas availability characterizing extra heavy oils. Surprisingly, publications of experimental data concerning gas diffusivity in heavy oils are relatively rare. Furthermore, published values can vary by different orders of magnitude. This is probably due to the fact that this kind of measurement is quite tricky. Indeed gas leakages in high pressure cells could be in the same order of magnitude than gas diffusion in oil. To study very viscous oil (extra heavy oils), corresponding to very low diffusivity, it is thus necessary to carefully analyze the obtained results. The aim of this work is to characterize experimental fickian diffusivity of methane in heavy oils using two different concepts in order to be able to validate the experimental results. In this paper we present the two different methods (thermophysics and analytical) we have settled. Experimental results clearly indicate that diffusivity in extra heavy oils is lower than usually considered for ‘conventional’ oil. From various experimental results we also propose a first mathematical model to simulate the influence of temperature on gas diffusivity.