Mo/ZSM-5 is one of the most studied and efficient catalysts for the dehydroaromatization of methane (MDA), but the mechanism of its operation remains controversial. Here we combine ab initio thermodynamic analysis with comprehensive mechanistic DFT study to address Mo-speciation in the zeolite and identify the active sites under the reaction conditions. We show that the exposure of Mo-ZSM-5 to the MDA conditions yields a range of reduced sites including mono- and binuclear Mo-oxo and Mo-carbide complexes. These sites can catalyse the MDA reaction via two alternative reaction channels, namely, the CC coupling (ethylene) and hydrocarbon pool propagation mechanisms. Our calculations point to the binuclear Mo-carbide species operating through the hydrocarbon pool mechanism to be the most catalytically potent species. Although all other Mo sites in the activated catalyst can promote C-H activation in methane, they fail to provide successful path to the desirable low molecular weight products.