Birefringent crystals with optical anisotropy, which are employed to modulate the polarization of light, play a vital role in many fields of the optical industry. In this Communication, two mixed alkali metal hydroisocyanurates, RbLi(H2C3N3O3)(2)center dot 2H(2)O (I) and CsLi-(H2C3N3O3)(2)center dot 2H(2)O (II), were synthesized by a simple aqueous solution method, and they feature layered structures composed of (2)(infinity)[H2C3N3O3](-) ribbons through hydrogen bonds with the separation of water molecules and alkali cations. They exhibit a simultaneously short ultraviolet cutoff and giant birefringence, resulting from the perfect parallel alignment of pi-conjugated (H2C3N3O3)(-) anions. Interestingly, the first-principles calculations elucidated that the electronic states and optical properties are slightly different owing to the distinct p-pi interaction between alkaline metals and the (H2C3N3O3)(-) group despite the isostructural crystallographic lattice.