Excessive use of arsenical pesticides has rendered cotton soils of Texas high in background arsenic concentrations, thereby potentially increasing human health risk. Health risks from arsenic are associated with forms of arsenic that are bioavailable to the human system. Therefore, an appropriate health risk assessment due to exposure to arsenic-rich soils requires an estimate of bioavailable arsenic, which is a function of its geochemical speciation. In order to examine the relationship between arsenic bioavailability and geochemical speciation of arsenic, a 1-year incubation study was performed. Soil speciation of arsenic was determined using a sequential extraction scheme and bioavailability was estimated using an “in-vitro” method that accounts for arsenic potentially available to the human gastrointestinal system. Two soil types were chosen: an organic Vertisol of the Houston Black series (a typical cotton soil of Texas) and a sandy Spodosol of the Immokalee series (minimal arsenic retention capacity, used for comparative purposes). The soils were amended with one inorganic and one organic arsenical pesticide at 3 rates: 45 mg/kg, 225 mg/kg and 450 mg/kg As. Concentrations of arsenic in the various soil phases and in the bioavailable fractions were measured immediately after pesticide application (0 time), after 6 months and 12 months. At 0 time, arsenic in Houston Black was predominately water-soluble. After 6 months incubation, water-soluble forms of arsenic decreased significantly; the decreasing trend continued with increasing soil aging as reflected by the 12-month data. The presence of high levels of extractable Fe/Al in Houston Black soil resulted in high arsenic retention, leading to decreased arsenic solubility. In contrast, forms of arsenic in Immokalee soil were predominately water-soluble at all times. A significant correlation was observed between soil speciation and in-vitro bioavailability of arsenic in both soils with the bioavailable fraction corresponding to the water-soluble and exchangeable phases of soil arsenic.