Effects of ionic strength on adsorption and desorption of phosphate in variable charge soils were investigated with batch method. Results indicate that when pH was higher than 3.7 or 4.0, the amount of phosphate adsorbed by red soil and latosol increased with increasing ionic strength, and when pH was lower a reverse trend was observed for both soils. Electrolyte affected the adsorption of phosphate by the soils through its effect on electrostatic potential of the ionic specific adsorption plane, which existed between Stern layer and diffuse layer. Zeta potential of soil colloids suggested that when pH was greater than the isoelectric point (IEP), the potential in the adsorption plane was negative and became less negative with increasing ionic strength, thus allowing increase in phosphate adsorption by the soils; when pH was less than the IEP of soil colloids, the potential was positive and decreased with increasing ionic strength, thus decreasing phosphate adsorption by the soils. The results of desorption of pre-adsorbed phosphate indicated that the amount of phosphate desorbed by deionized water was greater than that by 0.1 mol L^-1 NaNO₃. This was also ascribed to the effect of ionic strength on potential in the adsorption plane.