Inorganic arsenic (As) in drinking water is considered a human carcinogen with multiple sites of attack. There are numerous reports in the literature, based on past and ongoing experience in various countries in Asia and South America concerning the higher risks of skin, bladder, lung, liver, and kidney cancer that result from continued consumption of elevated levels of arsenic in drinking water. In ground waters, only inorganic arsenic in the form of arsenite [As III] and arsenate [As V] are significant. As III is much more toxic than As V. Moreover, due to the anionic character of As V in the drinking water, As V can be removed more easily than As III. Arsenic speciation of water samples changes over time, specifically, during transportation of samples to the laboratory and storage of samples for analysis due to redox reactions and photochemical oxidation. But, it is important to know the exact concentrations of the arsenic species in the drinking water at the source in order to develop proper treatment strategies, and for epidemiological studies. Therefore, arsenic-containing waters must be speciated immediately in the field or preserved for later speciation in the laboratory. Currently, there is no generally accepted (standard) method for field speciation or for preservation. This study describes the development of a reliable field-speciation method for inorganic arsenic species, and compares the new method with a recently developed USEPA preservation method.