The toxic impact of combined selenium (Se) and hexavalent chromium (Cr 6+ ) salts on activated sludge from a local wastewater treatment facility has been studied through respiration inhibition tests. Results indicated that Se had inhibitory effects at concentration values above 500 mg Se/L, while, Cr 6+ inhibited acetate biodegradation above a concentration of 0.05 mg Cr 6+ /L. The effects of the combination of Se and Cr 6+ on activated sludge was also found to be inhibitory for acetate biodegradation; 20% biomass respiration inhibition was achieved at the combination of 500 mg Se/L with 0.05 mg Cr 6+ /L. However, the biomass performance for acetate removal was enhanced by 10% using 50 mg Se/L combined with 0.05 mg Cr 6+ /L. The effect of heavy metals on microbial communities' metabolic activity in activated sludge was also investigated using EcoPlates TM . The results showed that the presence of Cr 6+ increased the activated sludge acclimation time for metabolic activity compared to the presence of Se. The highest and the lowest functional diversity were recorded for activated sludge without added heavy metals and activated sludge fed with Cr 6+ , respectively.