Oriented single-crystal Raman spectra of the caesium α-alums CsRh(SO₄)₂·12H₂O, CsRh(SeO₄)₂·12H₂O, and CsRh(SO₄)₂·12D₂O have been recorded at or below 80 K. The external modes of water co-ordinated to the mono- and ter-valent cations as well as the internal modes of S(Se)O₄^2– and [Rh{OH(D)₂}₆]³⁺ are found in the spectral region 275–1 200 cm^–1. A full assignment of the spectra in this region has been completed and the results are in close agreement with the predictions of factor group analysis. This work provides the first full vibrational characterisation of the Raman-active modes of the sulphate α-alum lattice. The site and factor group splitting of the XO₄^2– vibrational modes is independent of the identity of X but highly dependent on the alum type, with the internal modes of SO₄^2– in the caesium rhodium sulphate α-alums giving similar Raman band profiles to those of SeO₄^2– in the caesium selenate α-alums. The internal modes of [Rh(OH₂)₆]³⁺ are found at 548[ν₁(RhO₆)], 533 [ν₂(RhO₆)], and 315 cm^–1[ν₅(RhO₆)] in the low-temperature single-crystal spectra, while the ν₁(Rho₆) mode occurs at 529 cm^–1 in solution (300 K, 1 mol dm^–3 H₂SO₄). The relationship between the wavenumber of the ν₁(RhO₆) mode and the Rh–O bond length is significantly different compared with that obtained for the first-row transition-metal tervalent hexa-aqua ions.