The effect of pH on the stoichiometry and kinetics of acetate uptake by phosphorus-accumulating organisms (PAOs) was studied. The stoichiometry of glycogen consumption and poly-P-hydroxy-alkanoates (PHA) accumulation was independent of the pH over the range 6.5 to 8.0. It was again demonstrated that the amount of phosphorus released per acetate taken up (P/Hac ratio) was linearly dependent on pH, because of additional energy requirements for acetate transport at higher pH. The slope of this relationship was similar to that in previously published work, but the absolute values were different, indicating that the P/Hac ratio is the most variable stoichiometric parameter associated with the anaerobic metabolism of PAOs. A kinetic expression for acetate-uptake rate was developed and tested. it assumes a zero-order form when the polyphosphate content of the biomass is not limiting. When the polyphosphate content becomes low, the rate is significantly decreased. The expression was tested in situations in which polyphosphate was a limiting factor in the rate of acetate uptake, in which the glycogen content of the biomass became very low, and in which both glycogen and polyphosphate were present in excess. The model was able to simulate the three situations adequately. Additionally, the rate of acetate uptake was independent of the pH for the range studied (6.5 to 8.0).