Rapid encroachment of suburban development on former agricultural lands has greatly increased the potential for human exposure to arsenic (As), a group A carcinogen used extensively as pesticides prior to the 1990s. Recent studies have focused on the health risk posed by long-term human exposure to low-level As-contaminated systems, particularly due to soil ingestion from hand-to-mouth activity by children playing in the backyards. Many baseline risk assessments of As-enriched sites assume that all (100%) As present in the soil is bioavailable. This assumption seriously overestimates the actual risk (thereby increasing site clean-up expenses) since various geochemical forms of As are stable and/or insoluble in human gastric/intestinal juices and are not likely to be bioavailable. A laboratory incubation study is in progress to identify the relationship between geochemical speciation and “in-vitro” bioavailability of As in soils as a function of soil and pesticide properties. Five different soil types were chosen based on their potential differences with respect to As reactivity: an acid sand with minimal As retention capacity, a sandy loam with relatively high concentration of Fe/Al-oxides (hence, higher As retention capacity), a low-pH clay soil, an organic (muck) soil, and a high pH calcareous soil. The soils were characterized for pH, EC, moisture content, organic matter content, cation exchange capacity, particle size, extractable Mg, Ca and P content, and total Fe, Al and P content. The soils were amended with one organic (Dimethyl arsonic acid) and two inorganic As pesticides (sodium arsenate and sodium arsenite) at three rates: normal (45 mg/kg), above normal (225 mg/kg) and excessive (450 mg/kg). A sequential extraction scheme was developed to identify the various geochemical forms of As in pesticide-applied soils (soluble, exchangeable, organic, Fe/Al-bound, Ca/Mg-bound, residual). Concentrations of these operationally defined soil As forms will be correlated with the “in-vitro” bioavailable fractions to identify the As species that are most likely to be bioavailable under a variety of soil/pesticide scenarios. Results from this study will provide realistic starting points in site- and composition-specific human health risk assessment associated with long-term exposure to low doses of arsenic in soils.