The oxidation rate of arsenite in seawater is very slow. Of the parameters investigated which might affect this rate the most important were temperature, initial arsenite concentration, salinity and pH. An empirical rate expression was developed from experimental data which led to a prediction that the natural rate of oxidation in the ocean is about 0.023 micromoles of As(III) per liter each year. This value is about twice the natural concentration of arsenite; the only mechanism which seems able to account for the continued presence of arsenite is biological reduction of arsenate.