Impedance spectroscopy as a non-destructive evaluation technique was employed to study the microstructural evolution of thermal barrier coatings exposed to Na 2 SO 4 at 950°C. The results showed that the resistance and capacitance of yttria-stabilized zirconia top coat increased with corrosion time. The resistance of thermally growth oxide (TGO) from 20 to 60h decreased slowly, which indicates there is little change in the composition of TGO. The fast decrease in the resistance of TGO from 60 to 100h may correspond to the compositional change of TGO from a-Al 2 O 3 to more conductive NiO and Ni(Cr, Al) 2 O 4 with corrosion time.