The infrared absorption of materials like amorphous silicon and germanium shows that the atoms have first‐order effective charges although the corresponding crystals do not. This paper demonstrates that these charges originate in the second‐order effective charges of the crystals which are converted in the amorphous phases to first‐order charges by the distortion of the interatomic coordinates. The intensities of the amorphous‐phase fundamental spectra are therefore related to the intensities of the binary combinations of the crystals and the mean square displacements of the atoms required to form the amorphous phases, and they can be used to determine the displacements. For the same model, the difference of the second moments of the density of states for the amorphous and crystalline phases is a sum over some of the fourtorder potential constants. Literature spectra of silicon and germanium have been analyzed, and give root‐mean‐square atomic displacements of 0.59 and about 0.25–032 A˚ respectively.