This study has investigated in a comparative fashion the effect of cations and polyelectrolytes on the characteristics and final properties of both synthetic and activated sludge. Synthetic sludge was prepared according to established procedures; activated sludge was produced in a lab-scale, continuous-flow reactor which was fed with live activated sludge from a wastewater treatment plant. The results from experiments indicate that cations and polyelectrolytes could influence sludge floc characteristics as they relate to sludge conditioning. The relationship between polysaccharide concentration and cation concentration was examined at laboratory scale during flocculation with both synthetic and activated sludge; an increase in feed cation concentration led to a decrease in final polysaccharide concentration. An increase in cation concentration in the feed to the reactors was also associated with an increase in the bound biopolymer concentration. The changes in the bound biopolymer were explained according to the cation bridging model. The effects of a polyelectrolyte conditioner on sludge conditioning were determined. A qualitative link exists between polyelectrolyte conditioner dosage and sludge conditioning for both types of sludge. The two types of sludge also have very similar sludge conditioning after cationic polymer is added to the reactor. Thus synthetic and activated sludge behave very similarly in terms of their characteristics and sludge conditioning, and synthetic sludge can be used as a surrogate in activated sludge studies. The results of this work also indicate that the formation of cation-polymer complexes and polymer gelation are amongst the most important mechanisms for sludge coagulation-flocculation, and offer a means for optimisation of the activated sludge process.