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Phosphate enhances the mobility of As in well‐aerated soils by competing for adsorption sites. Phosphate and As may also coexist in large concentrations in hydric soils, and the influence of P on As in anaerobic systems is largely unknown. To determine the effects of P on As dynamics during a soil flooding and aeration cycle, samples of two soils were amended with Na₂HAsO₄ and Na₂HPO₄ and incubated under a N₂ atmosphere for 41 d, and then reaerated for 7 d. Subsamples were collected intermittently and dissolved As, Fe, Mn, Ca, S, P, and H₃AsO₃ concentrations were determined. Arsenic speciation in the soil solids was determined after 14 and 41 d of flooding and then after 13 h of aeration by X‐ray absorption near edge structure (XANES) spectroscopy. Arsenic sorption was small under anaerobic conditions, and H₂PO⁻₄ additions enhanced As(V) reduction rate in both soils and slightly suppressed As …