AbstractThe thermal diffuse scattering (TDS) of X‐rays in the anharmonic one‐phonon and harmonic two‐phonon approximations is considered. The complete expression for the one‐phonon intensity of X‐rays is derived by perturbation theory and presented for a polyatomic crystal in the high‐temperature limit. The method of evaluating the corrections of the X‐rays intensities for the thermal diffuse scattering is given and the importance taking into account the anharmonic effects in the one‐phonon scattering for KZnF3, CsCl, and YBa2Cu3O7−δ crystals at room temperature is discussed. Anharmonic TDS‐effects achieve considerable magnitudes for soft (acoustic) anharmonic crystals as CsCl.