Mo/H-ZSM5(0-8wt% Mo) catalysts for CH₄ aromatization were prepared from physical mixtures of MoO₃ and H-ZSM5. Surface diffusion of MoO_x species occurs at 623 K, and migration continues outside ZSM5 crystallites up to 773 K. If the Mo loading is higher than that which forms a single layer of MoO_x on the external surface of zeolite, Mo will be lost via sublimation of (MoO₃)_n oligomers or will destroy the ZSM5 framework by extracting Al to form inactive Al₂(MoO₄)₃ domains. Between 773-973 K, gas-phase transport of Mo also becomes important as Mo migrates inside zeolite channels, exchangers at acid sites and condenses to produce H₂O. Loss of H as H₂O during catalyst preparation and titration of remaining H⁺ sites indicates exchange of one H per Mo added to H-ZSM5, consistent with a proposed structure of (Mo_{2/O5)⁺² ditetrahedra located at two cation exchange sites.}