THE MEDIUM effect on the electronic absorption spectra of 2-(2-hydroxynaph-1-ylazo)-pyridine (HL) was studied. 2-(2-hydroxynaph-1-ylazo)-pyridine has been found to be a good chromogenic reagent for determination of Mn(II), Fe(III), Co(II), Ni(II) and Cu(II) ions. In aqueous buffer solution of different pH values (2-12) containing 30 % (v/v) methanol-water, HL reacts with Mn(II), Fe(III), Co(II), Ni(II) and Cu(II) ions to form 1:1 and 1:2 (M:L) chelates. The formed complexes exhibiting absorption maxima at 580, 555, 577, 561 and 551 nm with apparent molar absorptivities 2.42 x 10 4, 0.58 x 104, 2.14 x 104, 4.54 x 104 and 2.09 x 104 (L mol-1 cm-1) for Mn(II), Fe(III), Co(II), Ni(II) and Cu(II)-complexes, respectively. The optimum conditions (pH, time, temperature, reagent concentration, sequence of addition and solvent ratio) for chelates formation are studied. The spectrophotometeric analytical characteristics (linearity ranges, Ringbom ranges, stability constants and free energy changes) of the formed complexes are determined. Relatively large amounts of co-existing elements, including all other metals, can be tolerated. The advantages of this method are: simple and rapid, with high sensitivity and good selectivity and was applied to the determination of the mentioned metal ions in natural samples with satisfactory results. Some solid HL-complexes are prepared and studied by different analytical and spectral techniques in order to investigate the mode of bonding in the formed complexes.