The ability of solutions of phosphate and of hydroxide to displace previously adsorbed molybdate was investigated. In both solutions, displacement increased with increasing pH but the specific effect of phosphate was greatest near pH 7. Displacement was slow, especially after prolonged prior contact between soil and molybdate, and 48 hours’ shaking with phosphate solution did not displace all of the added molybdate. With increasing period of incubation of soil and molybdate, the proportion which was not displaced by phosphate increased. The conversion to a form which was difficult to displace was slow and, at 25°C, it appeared to be continuing after 12 months’ contact. The rate of conversion was increased by increasing temperature within the range 10°–40°C. The proportion converted was not affected by level of addition of molybdate within the range tested and it is suggested that this indicates that conversion does not involve diffusion in the solution phase of molybdate from one kind of site to another.