We present our electrochemical studies of epitaxial ruthenate and iridate films grown on single crystal substrates. Iridates and ruthenates are among the most studied oxygen evolution reaction (OER) electrocatalysts for water splitting applications. In the past decade, advances in deposition technologies and substrate availabilities have enabled the growth of single-crystal transition-metal oxides with high structural perfection. We use these advances, specifically Molecular-Beam Epitaxy (MBE), to prepare epitaxial ruthenate and iridate films on single crystal substrates. The use of epitaxial, single-crystal films allow us to link the features from our electrochemical characterizations to the physical adsorptions, which we will compare to the first-principle calculations. We further discuss the alloying effect on the surface adsorption process. We use this information to reveal insights into how to tune the surface physicochemical properties on the ruthenates and iridates and discuss the implications on the OER electrocatalysis.