In this work, the formation conditions of the red-brown (Δ=450 nm) complex which was formed from 3-4-diaminobenzensulfonic acid (DAB) and iron (III) were studied and stability constant determined spectrophotometrically. The acid constant of ligand was determined potentiometrically and found as pKa=3.413+-0.201 at 20oC. by using the Job’s method of continuous variation, the composition of complex was found as Fe(III)/DAB=3/2 at pH=4.00. From mole ratio curve were observed two inflection points for Fe(III)/DAB=1/2 abd 3/2. When was determined iron(III) in the solution was prepared as Fe(III)/DAB mole ratio equal to 3/2 titrated by standard EDTA solution was found that only one third of total iron at iron(III) oxidation state. Thus, two third of total iron oxidizes the equivalent amount of DAB to monoamine molecules. The formula of the soluble complex may be [Fe(DAB)2(H₂O₂)]+. The formation constants of complex were found as K=8.20.107 (log K=7.914) by using ligand excess method and K= (2.907±0.436)10⁷ (logK=7.464) by using equal absortive solutions method for 20oC and I=0.125. The free energy change in the formation of complex is ΔG=41.86 kj.mol^-1. Keywords :