The synthesis and analytical properties of phthalaldehyde. bisguanylhydrazone (PABGH) are described. The reagent was tested with 34-common cations, at several pH values, but only Co(II), Cu(II), Ni(II) gave colored reactions. The Co(II), Cu(II) complexes are formed in ammonia medium and only the Co(II) complex is stable in the presence of cyanide. The Ni(II) complex is formed in sodium hydroxide medium. Using these behaviors the Co(II)-Cu(II)-Ni(II) mixtures can be analyzed by spectrophotometry in the presence of the remaining tested cations.