The tetrachlorocuprates. of the 2-aminothiazolium ion and of four of its substitution products, of 2-aminothiazolinium, and of 2-amino-4-carboxyl-thiazolidinium ions were prepared from the cor-responding heterocyclic bases, hydrochloric acid and copper(II) chloride in ethanol. The thermo-chromic compounds were characterized by their IR and electronic spectra. The EPR spectra of solutions, frozen solutions, and of the solids show the CuCl 4 2- ion to have the structure of a flattened tetrahedron; in the case of some of the solids, distortions from D 2d symmetry are observed. This was confirmed by two X-ray crystal structure determinations. Crystal data: 2-amino-2-thiazolinium tetrachlorocuprate, monoclinic, space group C2/c, a = 2772.8, b = 780.0, c = 1560.0 pm, β = 112.04°, Z = 8 (R = 0.047 for 2527 observed, independent reflexions); 2-amino-4-methylthiazolium tetrachlorocuprate, triclinic. P1̄, a = 780.4, b = 853.4, c - 1381.4 pm, α = 103.50, β = 99.39, γ = 104.17°, Z = 2 (R = 0.041 for 3319 reflexions). In both compounds the flattened CuCl 4 2- tetrahedron is distorted and has two bond angles in the range between 130 and 141°. The heterocyclic cations are protonated at their ring N atoms; the corresponding H atoms and the H atoms of the amino groups are involved in N−H···Cl hydrogen bonding.