When polychromatic radiation is used in x‐ray diffraction, the instrumental broadening changes with the scattering angle. It is shown that when transmission geometry is used, the instrumental broadening can be expressed in terms of the profile of the primary beam and its wavelength distribution. Consequently, it becomes possible to correct the entire intensity profile for instrumental broadening. The correction is made by successive foldings of the observed profile, and is demonstrated using the intensity profiles of raw and heat‐treated carbon black.