Biodegradation of organic matter and nitrification of ammonia nitrogen was studied by measuring the electron transport system (ETS) activity in activated sludge. The feasibility of characterizing the bioactivity of activated sludge based on the ETS was discussed. Then, bioactivity kinetics for the biodegradation and nitrification of organic matter was analyzed using the Michaelis.Menten equation. The results indicated that the ETS activity of activated sludge reflects the progression of organic matter biodegradation and nitrification of ammonia nitrogen; moreover, ETS activity is sensitive to the loading of organic matter and ammonia nitrogen and also to changes in alkalinity during the reaction. Therefore, it is feasible to characterize the bioactivity of an activated sludge system with ETS activity. The Michaelis constant for organic matter biodegradation was KTs=368.9mg/L; UTm=90.9mgTF/(gTss·h); KIs=88.42mg/L; and UIm=277.8mgINTF/(gTss·h); for the nitrification of ammonia nitrogen, the Michaelis constant was KTs=16.89 mg/L; UTm=34.6mgTF/(gTss·h); KIs=6.0mg/L; and UIm=196.08mgINTF/(gTss·h). Additional analyses of bioactivity kinetics confirmed that the organic matter oxidation rate of heterotrophic bacteria was higher than that of autotrophic nitrifying bacteria.