This article evaluates effects of pH and substrate on the competition between glycogen (GAOs) and phosphorus accumulating organisms (PAOs). A sequencing batch reactor system was operated for enhanced biological phosphorus removal (EBPR) with acetate as the sole carbon source and acetate added domestic wastewater at different influent acetate/phosphate ratios. Some batch tests were performed using acetate added domestic wastewater at different influent acetate/phosphate ratios, with different initial pH values of acetate and domestic wastewater mixture. The resulting experimental data supported the presence of GAOs for all tested HAC/P ratios, especially under P limiting conditions for acetate as sole carbon source. Strong evidence is observed that acetate added domestic wastewater system had higher PAOs fraction than acetate system as sole source carbon, with using model components, namely substrate uptake, glycogen utilization and P release.