This research examined the biodegradability of a recalcitrant or inhibitory waste stream with the use of an electrotytic respirometer. Specifically, it addressed the oxygen uptake rates associated with biological treatment of pentachlorophenol waste‐water from a wood preserving operation. Results indicated that the respirometer can be successfully used to develop an acclimatization program for biological treatment of inhabitory waste streams. Based on a gradual introduction of the PCP wastewater, overall COD reductions of 71.4 percent were realized in a bench scale activated sludge treatment system.