In this study, the potential of alum sludge, obtained from a drinking water treatment plant located in Istanbul, Turkey, as an adsorbent for the removal of phosphate from aqueous solution was investigated. The effects of adsorbent amount, pH, initial concentration, and temperature on phosphate removal were studied by batch adsorption experiments. Equilibrium data were analyzed according to the Langmuir and the Freundlich isotherms. The Freundlich isotherm equation provided a better fit than the Langmuir isotherm equation. Thermodynamic parameters (Delta G, Delta H, and Delta S) related to the process were also calculated and reported. The positive value of Delta H and Delta S indicated the endothermic nature of the phosphate adsorption and the affinity of the adsorbent for the phosphate, respectively.