So far, anaerobic sulfate-dependent acetate oxidation at high pH has only been demonstrated for a low-salt-tolerant syntrophic association of a clostridium ‘CandidatusContubernalis alkalaceticum’ and its hydrogenotrophic sulfate-reducing partnerDesulfonatronum cooperativum. Anaerobic enrichments at pH 10 inoculated with sediments from hypersaline soda lakes of the Kulunda Steppe (Altai, Russia) demonstrated the possibility of sulfate-dependent acetate oxidation at much higher salt concentrations (up to 3.5 M total Na+). The most salt-tolerant purified cultures contained two major components apparently working in syntrophy. The primary acetate-fermenting component was identified as a member of the orderClostridialesforming, together with ‘Ca.Contubernalis alkalaceticum’, an independent branch within the familySyntrophomonadaceae. A provisional name, ‘Ca.Syntrophonatronum acetioxidans’, is suggested for the novel haloalkaliphilic clostridium. Two phylotypes of extremely haloalkaliphilic sulfate-reducing bacteria of the genusDesulfonatronospirawere identified as sulfate-reducing partners in the acetate-oxidizing cultures under extreme salinity. The dominant phylotype differed from the two species ofDesulfonatronospiradescribed so far, whilst a minor component belonged toDesulfonatronumthiodismutans. The results proved that, contrary to previous beliefs, sulfate-dependent acetate oxidation is possible, albeit very slowly, in nearly saturated soda brines.