Geochemical speciation of arsenic (As) in soils controls the degree of its bioavailability. Bioavailability of As in soils is typically an inverse function of soil retention. Factors that generally influence As retention in soils are soil pH, amorphous Fe/Al oxide contents, soil organic matter content, concentrations of P, Ca, and Mg, soil cation exchange capacity, and clay content. The objective of our study was to develop a comprehensive understanding of the relationship between geochemical speciation and bioavailability of As as a function of soil physico-chemical properties. As Phase-I, we conducted a laboratory study to identify the relationship between geochemical speciation and “in-vitro” bioavailability of As in soils as a function of soil and pesticide properties in a static, incubation system. Two different types of soils were chosen based on their potential differences with respect to As reactivity: Immokalee, an acid sand with low extractable Fe/Al, having minimal As retention capacity, and Millhopper, an acid sandy loam with high extractable Fe/Al oxides and hence, higher As retention potential. The soils were contaminated with sodium arsenate and dimethyl arsenic acid. The incubation study was followed by a greenhouse study in Phase-II involving dynamic interactions between pesticide, soil, water and plants under controlled environmental conditions. Rice was used as the test crop. Concentrations of the "operationally-defined" soil-As forms were correlated with the “in-vitro” bioavailable fractions to identify the As species that are most bioavailable. Results from this study helped elucidate the effect of soil properties on As biogeochemistry in chemically-variant pesticide-contaminated soils and identified those properties with the greatest influence on ultimate availability of As in the human gastrointestinal system.