The authors have carried out high-momentum-resolution studies in diffuse X-ray scattering of barium and lead B" aluminas in the temperature range 20-700 degrees C. They have also measured the vibrational spectra of these compounds between 2K and 300K in the energy range 10-100 cm-1. The results are discussed in terms of preferential occupation of a cation sublattice in the conduction slab of B" alumina and of cation clustering.