Electron spin resonance and Mössbauer spectroscopy were combined with chemical treatments such as acid hydrolysis, reduction with hydrazine and complexation with ammonium thiocyanate to obtain information on oxidation states and site symmetries of Fe bound by humic and fulvic acids. At least two, possibly three, different binding sites for Fe 3+ were found to occur in humic materials: (a) Fe 3+ was strongly bound and protected either by tetrahedral and/or octahedral coordination; this form of Fe exhibited considerable resistance to chemical complexing and reduction and was characterized by a distinct resonance at g = 4.1 and by Mössbauer doublets with I. S. = 0.18 mm/sec and 0.65 mm/sec and Q. S. = 0.67 mm/sec and 0.68 mm/sec, respectively; and(b) Fe 3+ was adsorbed on external surfaces of humic materials, weakly bound octahedrally, easily complexed and reduced, and characterized by a resonance at g ≈ 2 and by Mössbauer doublets with I. S. = 0.40 mm/sec and Q. S. = 0.60 mm/sec before reduction and I. S. = 0.94 mm/sec and Q. S. = 2.76 mm/sec after reduction. Laboratory-prepared Fe-fulvic acid complexes, containing between 5.5 and 50.1% Fe, showed that as more Fe was added, the intensity of the resonance at g ≈ 2 relative to that at g = 4.1 increased rapidly, indicating that most of the added Fe was bound to surface octahedral sites. Similar observations were made on a naturally occurring Fe-pan taken from a soil rich in organic matter. All of the Fe bound by the humic materials was found to be in the trivalent state.