The toxicity of kraft bleaching plant effluent to aceticlastic methanogens was investigated in batch assays, using an anaerobic serum bottle technique. The effluent was found to exhibit a strong inhibition of the anaerobic conversion of acetate to methane and carbon dioxide, expressed as a long lag-phase before methanogenesis started. The inhibitory compounds were shown to have a low molecular mass and to have their origin in the acidic part-stream of the bleaching plant effluent. The methanogens seemed not to be able to acclimate to the inhibitory compounds, but the toxicity was found to be bacteriostatic rather than bacteriocidal. The inhibitory compounds were found to be degraded slowly in the batch experiments, finally reaching non-toxic levels so that methane production could start. The toxicity of the chlorinated acetic acid analogues mono-, di- and trichloro-acetic acid were studied. These compounds were found not to contribute significantly to the methanogenic toxicity of kraft bleaching plant effluents.