The 60 nm-thick Ru(0001) layer was deposited epitaxially onto Al2O3(0001) by ultrahigh vacuum (UHV) sputter deposition at 500 degrees C followed by a step anneal (ex situ) to 950 degrees C. The Co layer was electrodeposited at room temperature from an acidic electrolyte (pH = 3.8) containing dilute Co metal ions. Previously unreported, evidence for the underpotential deposition (UPD) of Co on Ru(0001) is presented and was shown to affect the nucleation of Co during constant potential electrodeposition. This result demonstrates a strong interaction between Co and the Ru substrate necessary for epitaxial growth. Cross-sectional transmission electron imaging and diffraction confirmed the formation of an epitaxial layer of Co(0001) on Ru(0001) to practical thicknesses for interconnect gap-fill. This finding suggests the plausibility of electrodeposited, single crystal interconnects in future integrated circuit chips. (C) The Author(s) 2019. Published by ECS.