Selective UV-spectrophotometric methods for determination of iron(III) in iron(II) samples have been developed. The methods are based on the interaction of Fe(III) with quercetin and morin, compounds of the flavonoid group. Redox reactions occurring between Fe(III) ions and the reagents used make the basis for the detection. Iron(II) does not react with quercetin and morin under the conditions applied [aqueous-methanolic (3:2) soluions, 0.3 mol L(-1) HCl, 1.2 x 10(-4) mol L(-1) quercetin (morin)] and does not interfere with the determination of Fe(III). Iron(III) can be determined up to 15 microg mL(-1) using both the examined systems. The detection limits are 0.06 and 0.38 microg mL(-1) when using quercetin or morin, respectively. The method with quercetin was applied to the determination of Fe(III) (ca. 0.2%) in a Fe(II) pharmaceutical product.