This paper examines the role of short-chain volatile fatty acids in the excess biological phosphorus removal mechanism of the activated sludge process. The effectiveness of various substrate additions in inducing phosphorus removal was investigated through a series of laboratory and pilot-scale experiments. Phosphorus release and substrate uptake both take place in the anaerobic zone of the process and there appears to be an exchange phenomenon that occurs between the two molecules. The system phosphorus removal of the process is improved by the addition of sodium acetate to the anaerobic zone. It is important that the zone receives no incoming nitrate, as the added substrate will be oxidized in the denitrification reaction, rendering it unavailable for the phosphorus removal mechanism. Acetate and propionate, the two substrates that are most effective in inducing anaerobic phosphorus release, can be generated on-site at a treatment plant by primary sludge fermentation in concentrations sufficient to significantly enhance the phosphorus removal characteristics of the process.Key words: biological phosphorus removal, short-chain volatile fatty acids, phosphorus release, substrate utilization, primary sludge fermentation.