The crystal structure of alum K(Al_0.95Cr_0.05)(SO₄) ₂·12H₂O possessing anomalous birefringence was refined in space groups Pa3̄ and P1̄ (a = b = c = 12.165(2) Å, R = 0.0587). The distortions of cubic symmetry are attributed to the peculiarities of the orientational disorder in the distribution of SO₄ tetrahedra and to the different degrees of distortion of the aluminum octahedra.