Arsenic is widely distributed in the environment. The paper presents systematized data published about the thermodynamics of some relatively common arsenic oxysalts, which are formed in the weathering zone of the arsenide and sulfide ores, and determines approaches to quantitative physicochemical modeling of their formation conditions. The interpretations are summarized on the Eh–pH diagrams, synthesized from equilibrium calculations, and reported geologic occurrences. The most recent thermodynamic data available were used for the construction of diagrams from reactions which are balanced equations of Eh–pH relationships among species which are thermodynamically stable within the ranges of oxidation potential and pH considered for each reaction. The Eh–pH diagrams of systems, containing As and Fe, Cu, Pb, Co, Ni, Ca, were calculated and constructed using the Geochemist’s Workbench (GMB 9.0) software package. Eh–pH stability relationships have been determined for some widespread arsenic oxysalts (scorodite, mimetite, erythrite, olivenite, annabergite, beudantite, conichalcite, adamite, duftite) and in order to interpret conditions of formation of these minerals and to compare their geologic stabilities of ore deposits. The understanding of mechanisms of arsenic behavior in the near-surface conditions is one of actual problems of modern mineralogy and geochemistry and it is very important for the solving of some environmental problems.