In　order　to　investigate　the　metabolic　activity　of　microorganisms　and　the　rate　of　substrate　degradation　during　the　aerobic　biological　treatment,　a　method　of　on-line　determination　of　specific　oxygen　uptake　rate　(SOUR)　in　the　sequencing　batch　reactor　(SBR)　process　was　established.　The　trends　of　SOUR　in　the　organic　matter　degradation　and　ammonium　oxidation　processes　were　investigated.　It　was　shown　in　the　SOUR　profiles　that　the　system　underwent　three　stages　under　either　constant　DO　concentration　or　airflow　rate　conditions,　i.e.　deliquescent　COD　degradation,　ammonium　oxidation　and　endogenous　respiration.　The　heterotrophic　bacteria　showed　a　higher　DO　affinity　compared　with　the　autotrophic　bacteria,　thereby　resulting　in　COD　removal　firstly　and　subsequent　nitrification.　SOUR　was　0.36　mg　O2　·　(g　MLSS)-1　·　h-1　during　the　COD　degradation,　and　0.18　mg　O2　·　(g　MLSS)-1　·　h-1　during　the　ammonia　oxidation　at　the　constant　DO　concentration　of　1.0　mg　·　L-1.　SOUR　declined　sharply　when　ammonia　oxidation　was　completed,　which　indicated　completion　of　the　partial　nitritation　process.　Aeration　should　be　terminated　to　avoid　further　oxidation　of　nitrite　to　nitrate.　Accordingly,　initiation　and　maintenance　of　the　partial　nitritation　process　can　be　realized　by　identifying　the　slope　of　SOUR　profile.　©All　Rights　Reserved.