Iron(II) can catalyze the oxidation of luminol in seawater and this chemiluminescent reaction has been widely used for iron(II) determination. The method is vulnerable to interferences from other analytes that catalyze luminol oxidation. We have shown that addition of diethylenetriamine pentaacetic acid (DTPA) to a sample inhibits the reaction of iron(II) with luminol, while not affecting other substances that also catalyze luminol oxidation under our experimental conditions. DTPA-treated samples can therefore be used as sample blanks, with the difference between an untreated seawater sample and a DTPA-treated seawater sample related to the concentration of dissolved iron(II). The DTPA correction has been applied to measure diel variability of iron(II) due to photoreduction in a coastal environment, and to measure vertical distributions of iron(II) in the eastern tropical north Pacific oxygen deficient zone.