Abstract: To find the suitable tracers for activated sludge reactor flow pattern studies, the sludge adsorption experiment and aeration static settling experiment were performed with three alternative tracers, i.e. methylene blue, lithium chloride and sodium chloride. The adsorption effect of activated sludge on the 3 tracers and the impacts of tracer concentration on the activated sludge treatment efficiency were studied. The results showed that methylene blue was easily adsorbed by the sludge, leaving great influence on the experimental data, and the COD_Cr treatment efficiency by the sludge was reduced by up to 50% after adsorption of methylene blue. Lithium chloride and sodium chloride are ion-tracers which are difficult to be adsorbed; they will not affect the effect of activated sludge treatment. The treatment efficiency of COD_Cr remained at more than 70%. Finally, considering the tracer dosage and the effluent quality control, lithium chloride was identified as the most suitable tracer among the 3 alternatives.