Applications of nuclear magnetic resonance (NMR) to study a variety of physiological and biochemical aspects of bacteria with a role in the sulfur cycle are reviewed. Then, a case-study of high resolution 13C-NMR spectroscopy on sludges from bioreactors used for treating sulfate and sulfide rich wastewaters is presented. 13C-NMR was used to study the effect of sulfate and butyrate on propionate conversion by mesophilic anaerobic (methanogenic and sulfate reducing) granular sludge and microaerobic (sulfide oxidizing) flocculant sludge. In the presence of sulfate, propionate was degraded via the randomising pathway in all sludge types investigated. This was evidenced by scrambling of [3-13C]propionate into [2-13C]propionate and the formation of acetate equally labeled in the C1 and C2 position. In the absence of sulfate, [3-13C]propionate scrambled to a lesser extend without being degraded further. Anaerobic sludges converted [2,3-13C]propionate partly into the higher fatty acid 2-methyl[2,3-13C]butyrate during the simultaneous degradation of [2,3-13C]propionate and butyrate. [4,5-13C]valerate was also formed in the methanogenic sludges. Up to 10% of the propionate present was converted via these alternative degradation routes. Labeled butyrate was not detected in the incubations, suggesting that reductive carboxylation of propionate does not occur in the sludges.