The copper salts of the basic amino acids (arginine, histidine, lysine, and ornithine) are formulated as complex salts. They contain a cation (with two positive charges) consisting of a copper atom and two amino acid molecules. The anion can be a simple inorganic group, such as Cl−, NO3−, SO4=. The complex copper amino acid cations form double complex compounds with anions HgI3− or HgI4=. The double complex salts of arginine, [Cu(C6H14O2N4)2][HgI4], and lysine, [Cu(C6H14O2N2)2][HgI3]2, are only slightly soluble in water and have characteristic crystal forms. They can be used for the quantitative determination of these amino acids. The copper salts of the natural amino acids are classified into four groups.