Hardwood lignin (lignosulfonate) at 1.3, 2.6, and 3.9% (w/w) inhibited the acetate-utilizing methanogens of anaerobic digester sludge by 14.5, 18 and 21 days (in non-inhibitory conditions it took 10 days) to produce the same amount of methane. Methanogenesis under thermophilic (50 degrees C) conditions was completely inhibited by 2.6% hardwood lignin for 8 days regardless of the substrate (acetic acid) concentration. After that, at higher acetic acid concentration, more methane was produced. Increasing in the sludge concentration decreased the inhibitory effect of the lignin.