Infrared and ultraviolet-visible spectrophotometric study of pyridoxal isonicotinoyl hydrazone (PIH) show the existence of isomeric forms of PIH. The hydrazone / azonium tautomerization exists only in solid state. In solution the compound is readily converted to the hydrazone (normal Schiff-base) structure. Methanolic solution of PIH undergoes photo-isomerization under ultraviolet light, but in solid state this does not occur to any significant extent. The kinetic of the complex formation of Ni(II), Co(II), Mg(II) and Al(II) with PIH have been investigated in aqueous solution of ionic strength 0.30 M at 25.0 degree C. The reactions of divalent metal ions involve the formation of monochelated complexes obtained from stopped-flow spectrophotometric measurements. The rates of reactions are dependent on the hydrogen ion concentration and the pathways involving H3+L and H2L species are proposed for Ni(II) and Co(II), whereas HL- and L(2-) are proposed in the case of Mg(II). The mechanism of these reactions can be interpreted as being a dissociative interchange (Id) type. In contrast, the reaction between Al(III) and PIH is proposed to involve Al(3)+ and Al(,2)(OH)2(4) binding with H3L+. The stability constants of the complex formation between Ni(II), Co(II) and Mg(II) and PIH have been obtained from spectrophotometric measurements in aqueous solution of ionic strength 0.10 M at 25.0 degree C. It was shown that the major species present are Ni(H2L)(2+), Co(H2L)(2+) and MgL respectively.