Aims: To investigate the possibility of reducing excess sludge production in activated sludge processes by the addition of chemical uncouplers to greatly dissociate anabolism from catabolism. Methods and Results: Ortho-chlorophenol (oCP), 2,4-dichlorophenol (DCP), 3,3¢,4¢,5-tetrachlorosalicylanilide (TCS), para-dinitrophenol (pNP) and 2,4-dinitrophenol (DNP) were chosen for short-term tests for their ability to reduce sludge yield by shaking bottle test. The most effective chemicals, DNP and pNP, together with TCS were tested for various uncoupler concentrations and biomass concentrations. TCS was tested in a lab-scale completely mixed activated sludge batch culture. The model (demonstrated by Liu) was verified with experimental data in completely mixed activated sludge batch test, but was inconsistent with the results from the shaking bottle batch test. The observed growth yield (Y obs ) decreased with increasing of the ratio of initial uncoupler concentration to initial biomass concentration (C u /X 0 ). Conclusions: We suggest that the uncouplers oCP, DCP, TCS, pNP and DNP can cause a significant decrease in sludge production, the metabolism of which can explain the decline in sludge yield. Significance and Impact of the Study: The real strength of chemical uncoupler imposing on biomass should be C u /X 0 , not initial uncoupler concentration (C u ) alone. Chemical uncouplers can be used to develop the activated sludge processes for minimizing excess sludge production.