Allophane-free and -adsorbed humic acids were extracted from Ando soil with aqueous ammonia, and three kinds of fractions were sequentially isolated with 90% aqueous acetonitrile, 90% aqueous dioxane, and aqueous ammonia from the respective humic acids. The electron spin resonance spectra of each fraction was measured at pH 13.0 under argon atmosphere in the presence of various concentrations of Na2S2O4or K3[Fe(CN)6]. Spin concentrations of all the fractions increased as time elapsed after dissolution under argon. When air was bubbled into the solutions for 5 min, the concentrations were decreased by the oxidation with oxygen molecules. However, in time, the original signals were recovered. The spin concentrations were increased by the reduction with a small amount of dithionite, but decreased on increasing the concentration of dithionite. The spin concentrations were decreased by the oxidation with hexacyanoferrate(III), and new signals were observed with a large amount of hexacyanoferrate(III). When the amount of hexacyanoferrate(III) was not enough to oxidize all the electron donor moieties in the humic acids, the original signals were recovered over time. The data suggest that intramolecular or intermolecular electron transfer occurs in humic acids