A detailed analysis of the infrared reflectivity spectra of the superionic conductors X β-aluminates (X = Na, K, Ag, Rb and Tl) is presented. A Kramers-Kronig analysis and the oscillator fits to the data have yielded the frequencies of the transverse optic and associated longitudinal optic modes. A model which ignores the coupling between the spinel blocks accounts for the number of observed infrared modes as well as the Raman modes. It is shown that even for the modes characteristic of the motions of the disordered X-ions, the wave vector selection rule is still valid. The results are discussed with the available theoretical calculations. © 1978.