Arsenic is complexed with ferrihydrite in bidentate or monodentate structures in adsorption processes, which can reduce the effectiveness of As(V). Ferrihydrite is metastable and can transform into crystalline minerals with time, and this process could reduce the specific surface area, surface sites and change the adsorption properties (such as capacity, affinity) of arsenic. Previous studies have focused on the adsorption capacity of As(V) in ferrihydrite or crystalline iron (hydr)oxides (goethite, hematite and magnetite, etc.), however, the understanding of adsorption capacity of the different mass fractions and crystallinity iron (hydr)oxides produced by the ferrihydrite during the conversion process, the strong adsorption capacity of the crystalline iron oxides for arsenic, and the long-term stability of the formed iron-arsenic complexes is still lack. The objective of this study was to investigate the effects of arsenic concentrations on the transformation of ferrihydrite; the adsorption characteristics of different crystalline iron oxides on arsenic during the aging of ferrihydrite; and the stability of As(V)-Fe complexes at 75 °C. The time of adding arsenate was determined according to the conversion process of ferrihydrite, and the stability of As(V)-Fe complexes in different aging stages was investigated by wet chemistry experiments and spectroscopic analysis. Results as follows: (1) Arsenic reduced the transformation rate of ferrihydrite, and the inhibition intensity increased with arsenic concentration. High (Fe/As=1.875) and low (Fe/As=5.66) concentration of arsenate were added at different ferrihydrite aging period. The content of soluble Fe³⁺ remained basically stability in the high arsenic system but increased with culture time in low arsenic system, suggesting that the conversion and dissolution rate of ferrihydrite were affected by the arsenic concentration. (2) With ferrihydrite aging time, the release rate of arsenic was decreased and the stability of iron-arsenic complexes was increased, which affected by arsenic concentration. High-concentration and low-concentration arsenate were added at 0 d, initial stage (1d and 4d), middle stage (10d) and later stage (30d) of ferrihydrite aging, respectively. At 0 day of ferrihydrite aging, soluble As(V) contents began to increase after 1 day and 4 days mixing with high and low arsenate; at the initial stage, soluble As(V) contents of two treatment both began to increase after 8 day mixing; at the middle and later stages, soluble As(V) contents of two treatment both began to increase after 30 day mixing. (3) In the process of iron (hydr)oxide adsorbed arsenic, the stability of iron-arsenic complexes was affected by the crystallinity and surface characteristics of ferrihydrite and the competition between iron and arsenic for Fe₃ cluster sites. (4) XRD, FTIR and XPS analysis showed that partial arsenic gradually transferred from the surface of iron (hydr)oxide into lattice with the ferrihydrite aging and the arsenic adsorption time. Moreover, although ferrihydrite has a strong adsorption capacity for arsenic, the As(V)-Fe complexes is unstable and easily affected by the transformation of ferrihydrite.