Typical materials for applications in high-temperature, high-pressure aqueous solutions are stainless steels, nickel-base alloys, titanium, tantalum, noble metals, or ceramics. Under these conditions, these materials are often severely attacked, especially in the presence of acids, salts, or gases. Corrosion phenomena in high-temperature aqueous solutions are mainly influenced by the following factors: dissociation of acids, salts, and bases, the solubility of gases, the solubilities of corrosion products, and the stability of the protecting oxide layers. The influences of these factors on the corrosion, together with literature data of dissociation constants and solubilities of oxides, are reviewed in this paper. A recommendation is given for the application of possible reactor materials for oxidizing high-temperature aqueous systems in the presence of different inorganic species.