Octa- and heptachlorodibenzo-p-dioxins (OCDD and HpCDD) were formed during semi anaerobic digestion of sewage sludge at low temperature (20 degrees C). A twofold increase of OCDD and HpCDD was found after a digestion period of 192 days even if concentrations were corrected by corresponding accumulation factors caused by the degradation of biomass. Other polychlorinated dibenzo-p-dioxins and furans (PCDD/F) and polychlorinated biphenyls (PCB) were not increased. Under strictly anaerobic and aerobic conditions, however, no OCDD and HpCDD were formed. Semi anaerobic formation of OCDD and HpCDD may explain the prevalence of higher chlorinated PCDD in the homologues pattern typical for sewage sludge.