Co-based materials have emerged as promising ethanol steam reforming (ESR) catalysts in recent years. Both Co⁰ (Co metal) and Co²⁺ (CoO oxide) states were found to exist in the catalyst under reaction conditions and contribute to the catalytic activity, although their separate roles are still not fully understood. Density function theory (DFT) calculations were carried out to explore possible surface configurations based on the (100) and (111) facets of CoO. Ab initio atomistic thermodynamics was then applied to study the relative stability of various surface structures of Co⁰/Co²⁺ under ESR reaction conditions where H₂O and H₂ are the most abundant components in the gas phase. Based on the surface phase diagrams of CoO(100), CoO(111), and the general Co⁰/Co²⁺ catalyst, we found that the clean CoO(100) and OH*-covered CoO(111) are the most stable Co²⁺ surface configurations under ESR reaction conditions. We also suggest that a reducible support may be important in stabilizing Co²⁺ surfaces against reduction into metallic Co surfaces. The stable surface configurations of CoO identified in this paper can guide future DFT studies on the ESR catalytic activity of Co²⁺.