A highly sensitive and very simple spectrophotometric flow-injection analysis (FIA) method for the determination of iron(III) at low concentration levels is presented. The method is based on the measurement of absorbance intensity of the red complex at 410 nm formed by iron(III) and diphenylamine-4-sulfonic acid sodium salt (DPA-4-SA). It is a simple, highly sensitive, fast, and low cost alternative method using the color developing reagent DPA-4-SA in acetate buffer at pH 5.50 and the flow-rate of 1 mL min(-1) with the sample throughput of 60 h(-1). The method provided a linear determination range between 5 mu g L(-1) and 200 mu g L(-1) with the detection limit (3S) of 1 mu g L(-1) of iron(III) using the injection volume of 20 mu L. FIA variables influencing the system performance were optimized. The amount of iron(III) and total iron in river and seawater samples was successfully determined. Repeatability of the measurements was satisfactory at the relative standard deviation of 3.5% for 5 determinations of 10 mu g L(-1) iron(III). The accuracy of the method was evaluated using the standard addition method and checked by the analysis of the certified material Std Zn/Al/Cu 43 XZ3F. (C) 2008 Institute of Chemistry, Slovak Academy of Sciences.