Denuded zones in the near‐surface regions of Czochralski silicon wafers were investigated by means of both optical microscopy (OPM) and transmission electron microscopy (TEM). The results of this investigation show that the (nearly defect‐free) denuded zones formed near the wafer surface depend strongly on the initial oxygen concentration and the thermal history of the wafers, e. g., wafer annealing‐temperature sequence, gas ambient, and annealing duration. Clear denuded zones were observed in wafers annealed under conditions in which the oxygen out‐diffusion process is enhanced,e. g., in a high‐low, two‐step anneal (1050°C for 16h and 800°C for 16h), in a nonoxidizing ambient. A high initial oxygen concentration , using the old ASTM calibration factor (ASTM F121‐79), is required for the detection of a denuded zone, since defect generation in the bulk resulting from oxygen precipitation provides the demarcation layer.