The effect of sodium on the methanization of volatile fatty acid (VFA) mixtures was evaluated for three different sludges. Sodium concentrations causing 50% inhibition ranged from 3 to 16 g l(-1) in the absence of nutrients or other salts, showing a higher tolerance to sodium in the sludges obtained from the digesters treating high saline wastewaters. This fact is considered to be a consequence of sludge adaptation to sodium. Furthermore, the adaptation of methanogenic bacteria to sodium was also found during the batch assays : After 40 days of digestion, two different sludges (subjected to 6.9 and 21.5 g Na+ l(-1), respectively) increased the relative methanogenic activity from 0% to about 45% of the blank activity. The antagonism phenomena, due to the presence of cations and anions contained in the assayed media, greatly influenced the sodium effect on anaerobic sludges. Sodium concentrations causing 50% inhibition may be increased by 4 to 10 g l(-1) or more when sea water is used instead of NaCl. Nutrients also influence the sodium toxicity. The effect of sodium in each step of the anaerobic digestion process appears to be different depending on the sludge. For an adapted sludge, sodium caused 50% inhibition of propionic, acetic, and n-butyric utilizers at concentrations of 10.5, 17, and 19 g l(-1), respectively, an indication that propionic utilizers had less adaptation potential. These concentrations ranged between 5 and 6 g l(-1) for an unadapted sludge. The methanization of a more complex substance, such as glucose, was less extensively affected than the methanization of VFA. Results from continuous assays agree with those obtained from batch assays.