Arsenite (As III) precipitation in iron (Fe III) systems was firstly suggested from the shape patterns of the isotherms obtained for As(III) immobilized on hematite and goethite at pH 7.0. Hence, Raman and EXAFS analyses were combined with Density Functional Theory (DFT) to investigate the mechanism of As(III) immobilization on Fe(III) oxy-hydroxides. XANES analyses confirmed As(III) as the sorbed species. The results from DFT calculations indicated a stable structure for As(III) precipitated on hematite with the following calculated interactions: As = O (1.70 å); As-O (1.78 å); As-As (3.30 å), and As-Fe (3.50 å). These distances were in agreement with EXAFS analyses of As(III) sorbed on hematite (0.014 mmol/m2) and goethite (0.010 mmol/m2) at pH 7. The As = O bond was also consistent with the Raman vibration at 860 cm-1. Therefore, the precipitation of As(III) as a mechanism involved in As fixation by Fe(III) oxy-hydroxides is supported by spectroscopic analyses combined with theoretical calculations. This is invoked as the main finding of the present investigation and it may help the prediction and control of arsenic mobility in iron-rich soils.