Phosphate behavior is an important factor that controls the eutrophication of inland lakes. Dianchi Lake is the largest lake in southwest China, and it is divided into Caohai Lake (CL) and Waihai Lake (WL). Although the problem of eutrophication is more serious in CL than in WL, algal blooms are more frequent in the latter. In this study, phosphate-sediment interactions were studied with an emphasis on the differences between CL and WL sediments. A pseudo-second-order kinetic model could describe phosphate release kinetics well. The phosphate adsorption capacity of the CL sediments was thrice that of the WL sediments. Although phosphate release in these two lakes was comparable, the total phosphate concentration in the CL sediments was seven times that in WL sediments. Because phosphates and dissolved organic matter (DOM) compete for adsorption sites on the sediment particles, the presence of DOM results in a significant increase in phosphate release in both sediments. The results suggest that both endogenous and exogenous phosphates should be controlled in order to reduce algal bloom frequency in Dianchi Lake.