An optical ferric ion sensor, based on the use of 2,4-dinitrosoresorcinol (DNR) immobilized on XAD-7 and absorbance spectrometry, has been developed. To determine the sensor response to Fe(III) concentration, a kinetic approach is used, where the absorbance signal is measured at 10 min intervals. Reproducibility of Fe(III) measurements using the same probe (relative standard deviation RSD = 0.05%), and reproducibility of probe construction (RSD = 0.01%) are very good. Linear response is obtained for Fe(III) concentrations between 8.0 10 - 6 and 1.0 10 - 4 M with a detection limit of 8.0 10 - 6 M of ferric ion. The sensor is used successfully for the determination of Fe(III) in simulated seawater samples in the absence and presence of interfering ions. The results are comparable to those obtained by flameless atomic absorption spectrometry and relative errors are 1.30% and 0.98%, respectively, for the samples in the absence and presence of interfering cations by using the method described.