2,2',3,4-Tetrahydroxy-3',5'-disulphoazobenzene (tetrahydroxyazon 2S) has been synthesized for the first time. This reagent has been used for the spectrophotometric determination of aluminium and indium ions. The method is very sensitive and selective for the direct determination of aluminium and indium. The optimum pH and absorbance of complexes formed of tetrahydroxyazon 2S with aluminium and indium are 5; 500 nm and 495 nm for Al and In, respectively. The system obeys Beer's law at 0.05-1.6 mu g mL(-1) of aluminium and 0.06-2.1 mu g mL(-1) of indium concentration. The molar absorptivity is 6.42 x 10(4) L mol(-1) cm(-1) for aluminium and 7.70 x 10(4) L mol(-1) cm(-1) for indium. The molar compositions of the complexes are 1: 1 at optimum conditions. Alkaline arid alkaline earth elements, halogens, thiourea, ascorbic acid, Cd(II), Pb(II), Mn(II), Zn(II), Co(II), Ni(II). Cr(III), Bi(III), La(III), Si(IV) do not interfere this method. The method can be applied to the direct spectrophotometric determination of trace amounts of aluminium in steel, alloys, waste water, river waters, spring water and ground water. The method was also successfully applied to the indium determination in artificial mixture. (C) 2008 Elsevier B.V. All rights reserved.