Dissymmetrization of α-alum crystals of two isomorphous series, (NH₄,K)A1(SO₄)₂ · 12H₂O and K(A1,Cr)(SO₄)₂ · 12H₂O, with different values of anomalous birefringence has been studied by X-ray diffraction methods. Dissymmetrization is revealed mainly from the comparison of the intensities of crystallographically equivalent reflections in the space group Pa3̄ characteristic of alum. The degree of dissymmetrization depends on the position in which the isomorphous replacements occurs, as well as on the growth sector of the crystal, and shows a good correlation with birefringence.