The AISI 304 and AISI 10 18 steels (frequently used in solar collectors' plants) in contact with four different ionic liquids (ILs) suitable as diathermic fluids, were studied. Immersion tests were performed at 220 degrees C (the working temperature in such plants) for 10 days. The corrosion morphologies of the steels were investigated by scanning electron microscopy coupled with energy dispersive X-ray (EDX) microanalysis and the content of metals in the solution were detected via ICP-OES. The tests showed that the most performing IL is the ethyl-dimethyl-propyl-ammonium-bis(trifluoromethylsulphonyl)imide. The corrosion properties of the two alloys in contact with such IL were investigated by means of Tafel plots and resistance polarization at room temperature in open-to-air vessels. (c) 2007 Elsevier B.V. All rights reserved.