In earlier studies in the existing literature, concentration balance in the feeding solution has never been considered as an influencing factor when the studies were carried out under several feeding conditions to examine effects of different carbon sources on the enhanced biological phosphorus removal (EBPR) process. For a better understanding of a stable operation of an EBPR reactor, it is thought that effects of the concentration balance need to be combined and evaluated with effects of the type of carbon sources in EBPR studies. Therefore, this study was aimed at determining the effect of concentration balance on the performance stability and the phosphorus and glycogen dynamics of a sequencing batch reactor (SBR). In this study, the SBR operation was divided into two main stages. In the first stage, two different ratios of the total concentration of monovalent (M) to divalent (D) cations (in milliequivalent per liter, meq/L) (7.6 and 1.5, 30 days of operation for each) was applied in the feeding solution to investigate the effect of the concentration balance on the performance stability of the SBR. During this stage, sodium acetate was used as the sole carbon source. To investigate the effect of the type of carbon source on the EBPR process under the condition of constant M/D ratio, sodium acetate was used as the sole carbon source during the first half of the reactor operation of 120 days, and, then, the carbon source was abruptly switched to glucose in the second stage.