Soil ingestion by children via "hand-to-mouth" activity is the # 1 pathway of human exposure to arsenic (As) in As-contaminated residential properties. Arsenic-containing soil particles ingested by children are routed through the esophagus to the stomach where highly acidic and anaerobic conditions prevail. Such a harsh chemical environment might strongly influence the fate and stability of As-containing soil particles, leading to redox-mediated dissolution reactions. The extent to which soil-As is released into the gastric phase as well as its final oxidation state (arsenite is more toxic than arsenate) determines the magnitude of As absorption by human cells and the resulting toxicity. This study aims at understanding the fate and stability of As-containing soil particles in human stomach. We are particularly interested in evaluating potential changes in the oxidation state of As and the resulting As phases under typical gastric conditions. The human gastric phase has been simulated using the recipe provided by a widely used in-vitro gastric method (Ruby et al., 1996). In-vitro experiments to assess bioaccessible As are being conducted using (1) soluble As species (As(III), As(V), MMA, DMA), (2) crystalline and amorphous As phases (ferric arsenate, CCA-treated wood chips), (3) As contaminated soils collected from cattle dipping vat sites. Bioaccessible As is being speciated using a coupled HPLC-ICP-MS system. Seven soil samples have been obtained from former cattle dipping vat sites in Australia and five soils have been collected from Florida vat sites. Total As concentrations in these soils vary from 31 to 2143 mg kg-1.