In the present study freshly precipitated spent alum sludge [15-75 mg Al l(-1)] and alum [3-15 mg Al l(-1)] were tested and compared for their efficiency to remove phosphorus in synthetic wastewater [10mg P l(-1), 25 degrees C, pH=6] and it was proven that pure alum is much more efficient at phosphorus removal than the spent alum sludge. The effect of the pH on phosphorus removal efficiency was studied and it was proven that maximum orthophosphate removal is achieved in pH values 5-6 in both cases, alum and spent alum sludge, although alum is efficient in a wider pH range (4-7). The effect of aging [up to two months at 25 degrees C] on the efficiency of the spent sludge to remove phosphorus in synthetic wastewater was studied and it was proven that this efficiency is decreased through aging (up to 20% for 2 months aging). X-ray diffraction was used in order to examine whether this decrease is caused by changes in the structure of adsorbent and a mechanism was proposed to explain the above changes. Kinetics was investigated in the case of fresh and aged spent alum sludge and the pseudo-second-order chemisorption rate equation fits satisfactorily the experimental data [k: 0.286-1.048 g mmol(-1) min(-1), at 23-26 degrees C]. Finally, equilibrium was studied for fresh and aged spent alum sludge and the Freundlich isotherm fitted best the experimental data suggesting heterogeneous sorption [K(F)=14.4-19.7, N: 0.13-0.200].