Butyrate degradation in the presence of C-18 long chain fatty acids (LCFAs) was examined under anaerobic conditions at 21 degreesC. Butyrate degradation rates were a function of linoleic acid (LA) and oleic acid (OA) concentration but independent of the amount of stearic acid (SA) added. Within 2-4 h, butyrate reached undetectable levels in the control cultures. However, in cultures fed with LA, butyrate was removed within between 12 and 25 h and within 2-12 h for cultures inoculated with OA or SA. Propionate was detected in cultures fed with 50 mg dm(-3) LA and in cultures inoculated with OA and SA. LA exerted a greater inhibitory effect on butyrate-degrading organisms than OA and SA with longer removal times observed in cultures fed with LA. The propionate and acetate removal times and quantity produced were not related to the type and concentration of LCFA. (C) 2004 Society of Chemical Industry.