Alternative conditioning of activated sludge with seawater and brine was investigated with moisture and capillary suction time (CST) as the target index in this paper. Experiments have demonstrated that both the seawater and brine significantly improve sludge dewaterability, and the effect of the brine conditioning was similar to that of the seawater at double dosage. As the volume ratio of the brine to raw sludge was 1, the moisture dropped from 84.7 to 80.2% and CST decreased from 92.0 to 40.2 S. Response surface methodology (RSM) was utilized to analyze the combined effect of different cations on the sludge dewatering. The coexistence of Na+, Mg2+, Ca2+ and K+ was verified to play an important role in the conditioning process, and the order of conditioning effectiveness was as follows: Ca2+ approximate to Mg2+ > Na+ at the same equivalent concentration. (C) 2011 Elsevier B.V. All rights reserved.