Candidate materials for water wall of supercritical and ultra-supercritical utility boilers, T23 and T24, were chosen as the experimental samples and exposed to oxidizing atmosphere, reducing atmosphere and oxidizing/reducing alternating atmosphere separately. The corrosion temperature was 450-550 degrees C. The effects of oxygen content and temperature on the corrosion in reducing atmosphere and alternating atmosphere were investigated. The scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) were used to examine the corroded samples. The results show that the corrosion kinetics of T23 and T24 can be described by the double logarithmic equation and parabolic equation respectively. To describe the corrosion of materials accurately it is not sufficient to analyze the macro-mass gain and the macro-thickness of the corroded layer only, but the EDS should be applied to examine the migration depth of corrosive elements O and S. It is revealed that the corrosion becomes more severe when H2S is present in the corrosive gas. S is more active than O, and Cr can reduce the migration of oxygen but not S. The combination corrosion of S and O and pure [S] has a stronger corrodibility than pure H2S. T24 suffers the most severe corrosion at oxygen content of 0.8%. Corrosion is aggravated when the corrosion temperature is above 450 degrees C in the alternating atmosphere. T23 has better corrosion resistance than T24 and W contributes a lot to the corrosion resistance of T23.