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The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not enhance the activity of the surface Na 2 WO 4 catalytic active sites for CH 4 heterolytic dissociation during OCM. Contrary to previous understanding, it is demonstrated that Mn-promotion poisons the surface WO 4 catalytic active sites resulting in surface WO 5 sites with retarded kinetics for C–H scission. On the other hand, dimeric Mn 2 O 5 surface sites, identified and studied via ab initio molecular dynamics and thermodynamics, were found to be more efficient in activating CH 4 than the poisoned surface WO 5 sites or the original WO 4 sites. However, the surface reaction intermediates formed from CH 4 activation over the Mn 2 O 5 surface sites are more stable than those formed over the Na 2 WO 4 surface sites. The higher stability of the surface intermediates makes their desorption unfavorable, increasing the likelihood of over-oxidation to CO x , in agreement with the experimental findings in the literature on Mn-promoted catalysts. Consequently, the Mn-promoter does not appear to have an essential positive role in synergistically tuning the structure of the Na 2 WO 4 surface sites towards CH 4 activation but can yield MnO x surface sites that activate CH 4 faster than Na 2 WO 4 surface sites, but unselectively.