Redox reactions at metal oxide (MOx) surfaces are implicated in many catalytic and energy conversion processes involving proton-coupled electron transfer (PCET). Nonetheless, the fundamental thermodynamics dictating PCET reactivity at the MOx/solution interface are not well understood. Herein, we connect the pH-dependent electrochemical response of MOx thin films with the bond dissociation free energies (BDFEs) of their electroactive surface O-H bonds, using NiO as a case study. Complementary voltammetric and spectroscopic experiments show that the electrochemically determined BDFEs predict much of the observed PCET reactivity at the NiO surface. Analyzing the reactivity of MOx materials in terms of BDFEs could enable new approaches for designing more active and efficient (electro)catalysts for important PCET reactions.