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Humic substances (HSs), as electron shuttles, are associated with iron oxide transformation, yet the manner by which HSs affect Cd/As availabilities during this process under anaerobic conditions remains unclear. Two HSs (humic sodium, HA-Na, and biochemical fulvic acid, BFA) were applied at 0, 1, 2, and 4 g C kg−1 in a submerged incubation experiment. The dissolved, extractable and fractions of Cd/As and different iron oxides in soils were monitored. The addition of both HA-Na and BFA decreased the CaCl 2 -extractable Cd by 12.66–93.13%, and increased the KH 2 PO 4 -extractable As by 18.81–71.38% on the 60th day of incubation. The soil Eh and crystalline iron oxides (Fed) decreased, while amorphous iron oxides (Feo) and dissolved As increased after addition of both HSs. However, the two HSs had opposite effects on soil pH and dissolved Cd at the end of the incubation. HA-Na immobilized 19.47–85.99% more available Cd than did BFA over the incubation, although the extent of immobilization was similar with the maximum application rate on the 60th day. BFA mobilized 5.22–26.12% more available As than did HA-Na. XPS data showed that FeOOH decreased while the FeO component increased over the incubation. Correlation analysis and SEM showed that the reduction in the soil Eh and Fed and relative increase in Feo increased the available Cd, while decreased the available As. Consequently, the addition of HA-Na and BFA, particularly combined with flooding irrigation management, could effectively reduce the available Cd in Cd-contaminated soil. However, this method should be used with caution in As-contaminated soil. [Display omitted] • HA-Na & BFA effectively decreased CaCl 2 -Cd, and increased KH 2 PO 4 -As under flooding. • HA-Na & BFA promoted the transformation of crystalline Fe oxide to amorphous phase. • Influence of HSs on available Cd or As due to reduce of Eh and transformation of Fe. [ABSTRACT FROM AUTHOR]