In evaluating the quality of composts that include humic substances (HSs), the binding of iron(II) (conditional binding constants and binding capacities) are an important criteria. To determine the binding ability, it is necessary to determine the concentrations of free Fe(2+) and Fe(II)-HS complex species present. In the present study, we describe a colorimetric method for determining free Fe(2+) species using ferrozine in an aqueous solution of Fe(2+) and HS. It was found that the colored species in the Fe(2+) and HS mixture represented only free Fe(2+) species. Two water soluble fractions of HS, humic (HA) and fulvic (FA) acids, were isolated from a compost sample, and the conditional binding constants and binding capacities were estimated by colorimetric titration. Differences in the binding of Fe(II) to HA and FA are discussed, based on their structural features, which were characterized by acidic functional group analysis, elemental analysis and solid-state CP-MAS (13)C NMR spectral data. (C) 2010 Elsevier Ltd. All rights reserved.