Acclimated and unacclimated experiments were conducted with propionic/acetic acids wastewater and activated sludge (biomass) long-term cultured, respectively, with acetic and propionic acid enriched wastewater. Biomass acclimated with the propionic acid enriched wastewater, compared with unacclimated, had a increased specific soluble ortho-phosphate (SOP) release (increased from 0.75 to 0.91 mmol-P/g-MLVSS) and uptake (from 0.96 to 1.25 mmol-P/g-MLVSS), SOP-uptake/SOP-release ratio (from 1.28 to 1.38 mol/mol), net SOP removal (from 0.46 to 0.69 mmol/l) and SOP removal efficiency (from 61.33 to 92.00%). However, the ratios of both anaerobic accumulated and aerobic utilised polyhydroxyalkanoates (PHA) were reduced. The reasons for these changes were due to the decreased glycogen transformations and cell growth as well as increased phosphorus accumulating organisms (PAO) involved in the acclimated biomass. Further investigation indicates that the propionic acid enriched wastewater was less efficient for enhanced biological phosphorus removal (EBPR) than acetic acid in short-term tests, but much more efficient in the long-term cultured experiments. (C) 2004 Elsevier Ltd. All rights reserved.