The formation constant of iron(III) complex with glycine (Gly) ligand in aqueous acidic medium (0.2 M HNO[subscript 3], I = 0.2 M at 28 plus or minus 1 degree C) was determined spectrophotometrically in which a competing color reaction between Fe(III) and SCN[superscript -] was used as an indicator reaction. Under the specified conditions Fe(III) formed 1:1 complexes, [Fe(SCN)][superscript 2+] and [Fe(Gly)][superscript 2+], with both of these ligands. The molar absorption coefficient and the formation constant of the [Fe(SCN)][superscript 2+] complex were determined graphically as well as through dynamic optimization of six parallel equations using Solver tool in Excel. Values thus obtained were subsequently used to calculate concentrations of uncoordinated iron(III) and glycine in the mixed-ligand solution. The principal advantage of the proposed method is that it offers a simple and convenient approach for an undergraduate laboratory demonstration of quantitative comparisons of formation constants using an inexpensive spectrophotometer and Excel software that are readily available in most teaching laboratories. (Contains 2 figures and 2 tables.)