The vibrational spectra of the synthetic minerals crandallite and goyazite MAl3(OH)6(HPO4)(PO4) (M = Ca and Sr) were studied by means of vibrational (IR and Raman) and inelastic neutron scattering (INS) spectrometries at room and low temperatures. The results were discussed taking into consideration the effects of different types of disorder (random distribution of PO4 and HPO4, orientational distribution with HPO4 and displacement of Ca in crandallite). The consequences of the resulting perturbed symmetry and of the non-equivalence of the bridging µ-OH groups in the Al2OH fragments were pointed out (complexity of the ν(Al2OH), δ(Al2OH) and γ(Al2OH) bands). The vibrations of the PO4 and HPO4 groups could be separated. Some aspects of the vibrational behaviour of the AlO2(OH)4 units and of the layers composed therewith were addressed. The translational modes of the interlayer cations Ca2+ and Sr2+ could be observed; the findings support the suggested displacement of Ca in crandallite. Copyright © 2006 John Wiley & Sons, Ltd.