Oxygen uptake rate (OUR) experiments were performed with sludge from six laboratory-scale, continuously fed, activated sludge and PACT(TM) reactors (sludge ages of 4-, 8-, and 12-days) to evaluate the sludge's resistance to inhibitory compounds, Three inhibitory compounds with varied ability to sorb on activated carbon were tested: 3,5-dichlorophenol (3,5-DCP, strongly adsorbed), phenol (moderately adsorbed), and zinc (poorly adsorbed). The inhibitory compound concentration that reduced the unacclimated sludge's specific oxygen uptake 50% from its maximum rate was determined (IC50). For the organic compounds, PACT(TM) sludge resisted acute inhibition better for all sludge ages; sorption studies indicate that phenol sorbed onto the PACT(TM) sludge could account for the IC50 difference at the higher sludge ages. With 3,5-DCP, the 4- and 8-day-old PACT and activated sludge solids sorbed similar amounts of 3,5-DCP at concentrations near the IC50 values, yet the PACT(TM) sludge exhibited higher IC50 values; biomass differences may have accounted for the improved resistance to inhibition. With the poorly adsorbed zinc, no difference in IC50 or sorption was noted with the 4-day-old sludge. For the 12-day-old sludge, the PACT(TM) sludge was much more resistant to zinc exposure, with changes in the biomass rather than sorption on carbon the apparent reason.