Abstract The development of hypersonic vehicles (up to Mach 10) leads to an important heating of the whole structure. The fuel is thus used as a coolant. It presents an endothermic decomposition with possible coke formation. Its additional permeation through the porous structure involves internal convection. This implies very complex phenomena (heat and mass transfers with chemistry). In this paper, the n-dodecane pyrolysis is studied through stainless steel porous medium up to 820K and 35bar (supercritical state). The longitudinal profiles of chemical compositions inside the porous medium are given thanks to a specific sampling technique with off-line Gas Chromatograph and Mass Spectrometer analysis. By comparison with previous experiments under plug flow reactor, the conversion of dodecane is higher for the present experimental configuration. The pyrolysis produces preferentially light gaseous species, which results in a higher gasification rate for a similar pyrolysis rate. The effects of the residence time and of the contact surface area are demonstrated. The transient changes of Darcy's permeability are related to the coke formation thanks to previous experimental relationship with methane production. A time shift is observed between coke chemistry and permeability change. This work is quite unique to the author's knowledge because of the complex chemistry of heavy hydrocarbon fuels pyrolysis, particularly in porous medium.