The effects of dissolved organic compounds on the determination of nanomolar concentrations of Fe(II) have been compared using two luminol-based flow injection chemiluminescence (FI-CL) methods. One used the direct injection of sample into the luminol reagent stream, and the other incorporated on-line solid-phase extraction of the analyte on an 8-hydroxyquinoline microcolumn. The CL signals from analyses of dissolved iron species (Fe(II) and Fe(III)) with model ligands and organic compounds were examined in high-purity water and seawater. The organic compounds included natural reducing agents (e.g., ascorbic acid), nitrogen σ-donor/π-acceptor compounds (e.g., 1,4-dipyridine, protoporphyrin IX), aromatic compounds (e.g., 1,4-dihydroxybenzene), synthetic iron chelators (e.g., EDTA), and natural iron binding compounds (e.g., desferrioxamine B, ferrichrome A). Fe(II) determinations for both luminol FI-CL methods were affected by submicromolar concentrations of redox-active compounds, strong iron binding ligands (i.e., log KFeL > 6), and compounds with electron-donating functional groups in both high-purity water and seawater. This was due to reactions between organic molecules and iron species before and during analysis, rather than chemiluminescence caused by the individual organic compounds. In addition, the effects of strong ligands and size speciation on Fe(II) recoveries from seawater following acidification (pH 2) and reduction (100 μM sodium sulfite) were investigated. © 2005 American Chemical Society.