Propionate is an important intermediate in the anaerobic degradation of complex organic matter to methane and carbon dioxide. The metabolism of propionate-forming and propionate-degrading bacteria is reviewed here. Propionate is formed during fermentation of polysaccharides, proteins and fats. The study of the fate of 13C-labelled compounds by nuclear magnetic resonance (NMR) spectroscopy has contributed together with other techniques to the present knowledge of the metabolic routes which lead to propionate formation from these substrates. Since propionate oxidation under methanogenic conditions is thermodynamically difficult, propionate often accumulates when the rates of its formation and degradation are unbalanced. Bacteria which are able to degrade propionate to the methanogenic substrates acetate and hydrogen can only perform this reaction when the methanogens consume acetate and hydrogen efficiently. As a consequence, propionate can only be degraded by obligatory syntrophic consortia of microorganisms. NMR techniques were used to study the degradation of propionate by defined and less defined cultures of these syntrophic consortia. Different types of side-reactions were reported, like the reductive carboxylation to butyrate and the reductive acetylation to higher fatty acids.