Indirect determination of trace concentrations of iron was studied by monitoring the decrease of the a. c. polarographic wave of 1, 10- phenan tli rol i ne (phen). Fundamental harmonic, with current measured at Φ = 90°, was utilized. The best supporting electrolyte was found to be a 0.6 H NaClO4solution of pH 4.5 with acetate buffer. The approach consists in running polarograns for the supporting electrolyte containing buffer and phen, subsequent to sample addition, and after each addition of standard, respectively. The effect of pH, buffer system, some common anions and iron(III) was investigated. The method has been applied to the determination of iron in pure samples, reference standard material, and tap water. The detection limit was 4 ng mL−1.