Physiologically distinct anaerobic microorganisms were explored for their ability to oxidize different substrates with humic acids or the humic analogue, anthraquinone-2,6-disulphonate (AQDS), as a terminal electron acceptor. Most of the microorganisms evaluated including, for example, the halorespiring bacterium, Desulfitobacterium PCE1, the sulphate-reducing bacterium, Desulfovibrio G11 and the methanogenic archaeon, Methanospirillum hungatei JF1, could oxidize hydrogen linked to the reduction of humic acids or AQDS. Desulfitobacterium dehalogenans and Desulfitobacterium PCE1 could also convert lactate to acetate linked to the reduction of humic substances. Humus served as a terminal electron acceptor supporting growth of Desulfitobacterium species, which may explain the recovery of these microorganisms from organic rich environments in which the presence of chlorinated pollutants or sulphite is not expected. The results suggest that the ubiquity of humus reduction found in many different environments may be as a result of the increasing number of anaerobic microorganisms, which are known to be able to reduce humic substances.