The effect of temperature on the corrosion inhibition of low carbon steel in 31.3 g l −1 NaCl solution has been investigated. A mixture of aluminium sulfate and thiourea (A + T) was found to be an effective corrosion inhibitor at temperatures <50°C, but both A + T and A alone accelerate corrosion at temperatures >60°C. Pits formed in the presence of A alone at both high and low temperatures. The film formed on unhibited steel, identified by infrared spectroscopy, was lepidocrocite (γ-FeOOH). The film formed in the A and A + T systems was a form of hydrous aluminium hydroxide containing weakly bound molecular water with the structure similar to pseudo-boehmite. X-ray diffraction showed that at low temperatures the surface layer produced in A was in amorphous form and that in A + T possesses the crystalline characteristics of υ-Al 2O 3. From the results of pH monitoring on the steel surface and in the bulk solution, the possible mechanisms for the dual character of A + T have been proposed. The inhibition of A + T at low temperatures is due to the formation of crystalline υ-Al 2O 3 which provides a corrosion barrier; the acceleration of A + T at high temperatures is not only due to the non-protective film which has been peptized in a charged amorphous gel, but is also due to the extra attack of the acid, presumably induced by the dissociation of water and the hydrolysis of polynuclear aluminium hydroxide complexes.