The impact of chemical phosphorus removal by co-precipitation has only been considered from an effluent quality point of view. This paper considers the direct impact of two aluminium-based chemicals-aluminium sulphate and an aluminium-based polyelectrolyte compound-on an activated sludge pilot plant. Average chemical oxygen demand (COD) removals were 90.7% for the polyelectrolyte and 88.3% for aluminium sulphate. Nitrification appeared to be inhibited by the addition of high doses of aluminium sulphate. Ammonia removals of just 33.8% were observed compared with 95.4% for the polyelectrolyte. In terms of the direct effect on biomass, the chemicals differed widely. The oxygen uptake of the polyelectrolyte compound was on average 120% higher than that of aluminium sulphate, the latter averaged 0.0046 mgO(2)mg biomass(-1) hr(-1). Species diversity (ST) was significantly higher with polyelectrolyte addition (S-T = 30.5) than with aluminium sulphate (S-T = 23.3). There was also a large difference in the flee characteristics of the biomasses.