The possibility of the formation of PCDDs and dechlorination of PCDFs during the anaerobic digestion of sewage sludge in laboratory scale digesters was investigated. Digesters were spiked with 1,2,3,4,6,7,8-HpCDF–13C6 (240 ng/g organic matter (OM)) and pentachlorophenol (PeCP)–13C6 (24 μg/g OM) and the output sludge monitored for 60 days. No dechlorination or formation of the labelled or native PCDD/Fs was observed. The detectable 1,2,3,4,6,7,8-HpCDF–13C6 dechlorination yield was 0.0008–1% depending on homologue group and the detectable formation of OCDD yield was 0.00042% PeCP–13C6. Preferential respiration of other, more bioavailable, substrates is suggested as the most plausible explanation for the lack of dechlorination. Formation of PCDDs from PeCP has been observed in aerobic environments but this study provides further evidence that it is not a precursor for formation during anaerobic digestion.