To more precisely probe whether proton shuttling could be in play, we turned to proton inventory studies, an experimental technique that elucidates this behaviour by measuring catalytic activity as a function of electrolyte deuteration. Here, the relative activity of a catalytic system in a deuterated solvent, which is measured through current density, jn, is plotted relative to its activity in a fully protonated solvent, j0. Thus, jn/j0 can be modelled through a modified Gross-Butler equation: