The Mössbauer spectra at ambient temperature and 80 K of ⁵⁷Fe-humic acid complexes,of which were centrifuged or electrodialyzed,were recorded respectively,and the nature of Fe³⁺,Fe²⁺ bound with humic acid at different pH values was studied.The Mössbauer spectra and parameters of ⁵⁷Fe-humic acid slurry at pH 3 fitted by computer show that three doublets(Fig.1 AA',BB",CC') are reasonable.The Fe³⁺ in the humic acid at pH 3.0 occurs in more than one type of enviroment and high spin forms.The Mussbauer spectrum signal for Fe³⁺ was observed but not for Fe²⁺ in the ⁵⁷Fe-humic acid at pH 1.0.59% of iron doped was detected in the decantate at pH 1.0.It showed that considerable proportion of Fe³⁺ was reduced to Fe²⁺ by humic acid at pH 1.0,but no Fe²⁺ bound with humic acid strongly.The Mössbauer spectra for Fe³⁺,Fe²⁺ were found in the ⁵⁷Fe-humic acid samples (pH 2.8) which were electrodialyzed at 30 V.This indicates that a part of Fe³⁺ is reduced to Fe²⁺.Owing to the decrease of pH of ⁵⁷Fe-humic acid complexes suspension in the process of electrodialysis,considerable proportion of Fe²⁺ was bound with humic acid at pH 2.8 strongly.According to Mössbauer spectra recorded at 80 K,there is evidence for magneticallyordered component in the range of the pH 1-3 but no evidence for that at ambient temperature.