The corrosion of tantalum was investigated in sub- and supercritical oxidizing solutions of hydrochloric, sulfuric and phosphoric acid at temperatures between 360 and 500 degrees C. The corrosion rates in HCl and H2SO4 increased strongly above the critical temperature of water, which was attributed to a phase transformation from vitreous to crystalline Ta2O5. Corrosion rates in H3PO4 were low at all temperatures due to the formation of a top phosphate layer.