The degradation of volatile fatty acids by flocculant mesophilic (35 +/- 2 degrees C) syntrophic sludge grown in an anaerobic hybrid reactor fed with a mixture of propionate, n-butyrate and ethanol (1:1:2 on COD basis) at a volumetric loading rate of 3.7 kg COD per m(3) per day was examined. The propionate degradation rate amounted to 1.1 mmol per g volatile suspended solids per day. The same propionate degradation rate was measured in the presence of 10 mM acetate, but it decreased by 30% when 10 mM n-butyrate was added. C-13-Nuclear magnetic resonance spectroscopy demonstrated interconversion of 1% to 10% of both the propionate and butyrate pool, during their simultaneous degradation. Propionate formation from [2-C-13]butyrate was not via a direct decarboxylation. C-13-Propionate was converted via at least three different pathways. The first pathway was syntrophic conversion of propionate via the randomising pathway, evidenced by scrambling of [3-C-13]propionate into [2-C-13]propionate. Secondly, reductive carboxylation occurred, i.e. [3-C-13]propionate and [2,3-C-13]propionate were partly (2 to 10%) converted into [4-C-13]butyrate and [3,4-C-13]butyrate, respectively. Reductive carboxylation probably involved a transcarboxylase, as C-13-bicarbonate was not incorporated in the carboxyl group of butyrate. Thirdly, propionate was converted into higher fatty acids: [2,3-C-13]propionate was converted into [4,5-C-13]valerate and 2-methyl[2,3-C-13]butyrate.