Conducting physical and chemical tests with activated sludge is difficult because of its constantly changing properties. To overcome this difficulty a synthetic sludge that physically and chemically resembles activated sludge was created. In this study, the physical properties of synthetic sludge were examined and compared to those of activated sludge, including particle size, filterability by specific resistance to filtration and capillary suction time, conditioning with two polyelectrolytes, flee strength, rheological properties, and residual turbidity after settling. Results of this study show striking similarities between activated sludge and synthetic sludge. Filterabilities of both sludges are almost identical. A medium-molecular-weight, medium-charge-density, cationic polymer is effective in conditioning both sludges at approximately the same dose. Activated sludge is found to have stronger flocs than synthetic sludge. However, both sludges seem to have flocs stronger than some loose and weak flocs reported in the literature. Rheological behavior in terms of apparent viscosity of the two sludges is similar. The average particle size of activated sludge is found to be slightly larger than that of synthetic sludge. Also, synthetic sludge has a higher residual turbidity than activated sludge. These differences are explained by the fact that the synthetic sludge is lacking filamentous microorganisms that form the backbone of a flee structure in activated sludge. Even though there are some differences in properties, because its overall behavior is close to that of activated sludge, synthetic sludge can be used as a surrogate material for activated sludge when reproducibility in testing is necessary.