Alumina is resistant against corrosive aqueous solutions and could be used as a reactor material in the Supercritical Water Oxidation (SCWO) process. For this reason, the corrosion resistance of alumina and zirconia toughened alumina (ZTA) ceramics was investigated in aqueous solutions containing 0.1 mol/kg H2SO4, H3PO4 or HCl at T = 240 degreesC-500 degreesC at p = 27 MPa. In sulfuric acid, the solubility of alumina and its corrosion products was high at temperatures of 240 degreesC-290 degreesC. The corrosion rate was still high at higher temperatures (340 degreesC-500 degreesC), but the corrosion products were less soluble and formed a non-protecting scale on the samples. Phosphoric acid was less corrosive due to the formation of berlinite (AlPO4) on the surface of the specimens. In hydrochloric acid, the dissolution of the alumina grains was the predominant corrosion phenomenon at temperatures of 240 degreesC-290 degreesC. At higher temperatures, intergranular corrosion was observed, but a dissolution of the grains did not occur.