Polyoxometalate clusters provide well-defined atomic connectivity, cluster size and compositional flexibility, thus permitting studies of structure-function relations in oxide catalysis. These studies require that we determine the accessibility of active protons or metal-oxo species during catalysis and the kinetic relevance of steps involved in catalysis. Here, we report the effects of central atom (X), addenda atom (M), charge-balancing cations (Me), and secondary structure on the reactivity of protons in Me^m+_yH_8-n-myXⁿ⁺M₁₂O₄₀ clusters supported on SiO₂, for the specific case of 2-butanol dehydration catalyzed by Brønsted acids. This reaction proceeds via carbenium ion pathways limited by cleavage of C-O bonds to form butoxides with concurrent quasi-equilibrated formation of unreactive dimers. The number of protons accessible to reactants is measured by titration with 2,6-tert-butylpyridine during catalysis. The rate constant for butoxide formation and the equilibrium constant for dimer formation increased as proton acidity increased with increasing central atom valence, which led to a decrease in the number of charge-balancing protons, and also when H⁺ replaced more electropositive Cs⁺ cations. These parameters were similar for internal and external H⁺ protons in secondary structures and decreased with increasing charge on the anion.