Arsenic (As) is a human carcinogen, posing serious human health risk at concentrations as low as 10 ppb in drinking water. Elevated As concentrations in soils with a prolonged history of arsenical pesticide application, which are currently developed for residential purposes in metropolitan areas tremendously increases the probability of human exposure to As. Arsenic toxicity and carcinogenicity to living organisms is primarily due to exposure to the reduced species of As - arsenite, i.e., As(III), rather than the oxidized species - arsenate, i.e., As(V); the mobility of As(III) is significantly higher than that of As(V). One of the popular methods to decrease the mobility of As in soil-water systems is by promoting its retention onto amorphous Fe/Al hydroxides. Drinking-water treatment residuals (WTRs) are an inexpensive source of amorphous Fe/Al hydroxides. WTRs are waste products of drinking-water treatment, which are typically characterized by their high positive charge and have tremendous affinity for oxyanions (e.g., phosphate, arsenate). We hypothesized that the use of WTRs in arsenical pesticide-amended soils would decrease As(III) lability, even in the presence of phosphate fertilizers. An incubation experiment was set up using the State soil of Texas (Houston Black series), which has been extensively used for cotton production. Cotton plants have been traditionally treated with arsenical pesticides as defoliants. Two rates of As(III) pesticide were used (45 and 450 mg kg-1), and two P-fertilizer (TSP) rates - one was the P-based agronomic rate and the other was the N-based rate. The Fe- and Al-WTRs were mixed with the soil at the rates of 2.5 and 10% by weight. Subsamples were collected in triplicates at time zero and after 5 mo of incubation and were extracted by a typical sequential As and P fractionation scheme; they were also analyzed using an in-vitro As bioaccessibility method.