High‐temperature x‐ray diffraction has been carried out on a superconducting single‐phase sample of pseudo‐tetragonal Bi2Sr2CuO6+y, which was obtained from crystallization of thin glassy platelets. The sample displayed (00l) preferred orientation, which made it possible to determine the incommensurate modulation by conventional θ−2θ x‐ray scans. The existence of the modulation peaks (800 °C) indicates that the modulation cannot be caused solely by oxygen ordering. As temperature increased, it was observed that a decrease in the intensity of modulation lines was associated with an increase in intensity of fundamental reflection, which was similar to the order‐disorder phase transition. By removing 0.045(5) oxygen atom per formula unit, the components of modulation satellites along the b∗ and c∗ directions changed from bmod=0.183(9) and cmod=0.315(4) to 0.184(6) and 0.260(4), respectively. This demonstrates that oxygen, while not the sole cause, does play a role in the formation of the structural modulation. However, the invariance of bmod with respect to the change in oxygen content does not support the model which explained the modulation by inserting extra oxygen in BiO plane.