The ability of solutions of arsenate to displace previously adsorbed phosphate from soil was investigated. Displacement increased with concentration of arsenate, with time of shaking, and with pH. There was a marked change in slope of the pH-displacement curve near the pK2of arsenic acid. This was taken to indicate that arsenate was competitively displacing specifically adsorbed phosphate. At equivalent concentration the effectiveness of arsenate decreased in the sequence: Lithium, sodium, potassium. These differences may be associated with differences between the cations in their average distance of approach to a negatively charged surface. With increasing time of contact between soil and phosphate, the proportion of the added phosphate which was not displaceable by arsenate slowly increased. The rate of conversion to a nondisplaceable form increased with temperature. The conversion occurred over a wide range of soil moisture levels. The proportion converted was not affected by level of addition within the range tested, and it was argued that this indicates that conversion does not involve diffusion of phosphate from one site to another through the solution phase.