This study compared the PHAs production behavior of sludges from the anaerobic and oxic phases of an enhanced biological phosphorus removal (EBPR) system. This was accomplished by using the kinetics and stoichiometric coefficients obtained from aerobic batch tests to evaluate the performance of these two sludges. Experimental results indicated that the metabolic behavior of the sludges for PHAs production depend significantly on the operating sludge retention time (SRT) of the EBPR system. The oxic sludge with 5. days of SRT exhibited better PHAs production performance than anaerobic sludge. Conversely, the anaerobic sludge with 15. days of SRT had superior PHAs production capability compared to oxic sludge. These comparisons suggest that whether anaerobic or oxic sludge should be employed for PHAs production depends mainly on the operating SRT of the EBPR system.