explored extensively in many industrial applications, e.g., as heterogeneous catalysts, [2] energy storage devices, [3] or biomedical devices. [4] In particular, a major promising application is the development of hematite as a catalytic electrode material for photo-electrochemical (PEC) water splitting systems, [4,5] where photoexcitation leads to the evolution of O 2 and H 2 , thereby directly converting solar energy into chemical energy. In PEC cells, the oxygen evolution reaction (OER) involving (de)hydroxylation and (de)hydration occurs at the hematite/water interface. The local geometric and electronic structures of the hematite surface interacting with water/hydroxyl complexes are expected to play a significant role in determining the material catalytic activity, as these can largely impact the reaction thermodynamics and kinetics for each step of the OER mechanism [6] as well as the charge separation process. [7] Thus, it is critical to gain a detailed understanding of the surface configurations of hematite in contact with water in order to assist the development of efficient hematite-based PEC catalysts. In addition, water being one of the most pervasive molecules in the environment, it is an excellent probe to study substrate properties with regard to redox processes, adsorption capacity, impact of defects, and electronic corrugations in the presence of many other chemical species. [8] Adsorbed water can bind to the oxide surface in a molecular fashion or dissociative fashion (as OH − and H + ), through a variety of mechanisms including electrostatic interactions, charge transfer, or hydrogen bonding. Since a water molecule and its dissociative products possess markedly different chemical natures, the first step to describe many water-associated chemical processes is to establish the fundamental understanding of how water molecules and hydroxyls arrange on a given crystal surface. Despite advanced experimental techniques for surface characterization, it is not a trivial task to unveil the details of the surface configurations; one of the main challenges usually originates in the difficulties to accurately distinguish H 2 O from OH/O on the surface, especially when they are connected in a complex hydrogen-bond network. To shed light on this important issue, we rely here on a theoretical approach Hematite (α-Fe 2 O 3 ) is widely used as a catalytic electrode material in photoelectrochemical water oxidation, where its surface compositions and stabilities can strongly impact the redox reaction process. Here, its surface configurations in environmental or electrochemical conditions are assessed via density functional theory (DFT) calculations conducted at the Perdew, Burke, and Ernzerhof (PBE)+U level. The most energetically favorable surface domains of α-Fe 2 O 3 (0001) and (1102) are predicted by constructing the surface phase diagrams in the framework of first-principle thermodynamics. The relative surface stabilities are investigated as a function of partial pressures of oxygen and water, temperature, ...