The corrosion rates of AISI 316L and AISI 321H austenitic stainless steel, immersed in a stagnant isothermal mixture of 60% NaNO3 and 40% KNO3 molten salt at 550 degrees C in atmospheric air are 8.6 and 9.0 mu m/yr, respectively. The corrosion mechanism was proposed by recording the weight changes of the steel coupons at different time intervals up to 3000 h, and by the characterization of multilayer oxide scales formed on the steel surface. Multilayers made of different oxides, mainly Fe2O3 and Fe3O4, are the principal scale products. At 3000 h, the thickness of the scale layer formed on AISI 321H (7.5 +/- 2.9 mu m) is slightly higher than the one formed at the AISI 316L (6.9 +/- 2.1 mu m). This small difference might reflect the partial spallation of the corrosion layer on AISI 321H, which is seen for times longer than 1000 h. A minimal change of the composition of the molten nitrate salt is observed in time and is predominantly due to the appearance of soluble chromate products and nitrite compounds (0.004 wt% and 1.4 wt% at 3000 h, respectively). The observed corrosion behaviour of these alloys shows that they are good candidate for usage as containers of molten nitrate salts in the thermal energy storage (TES) system for a CSP plant.