Protonated ferrierite shows superior activity and selectivity in the liquid-phase isomerisation of linear unsaturated fatty acids to (mono-)branched-chain unsaturated fatty acids, (Mo)BUFA. This high selectivity is remarkable, as most of the interior surface of the zeolite is blocked already at the onset of reaction, limiting reaction to the pore mouth. A detailed study of the relationship between ferrierite acidity and performance is reported for five commercial catalysts; significant differences were found, independently of their bulk Si/AI ratios. Initial pore conversion correlates with Bronsted acidity in the 10-MR channels, as determined by adsorption/desorption of pyridine and FTIR. A low density of external acid sites reduces oligomerisation of fatty acids, while a high ratio of Bronsted to Lewis sites explains the observed high BUFA yield. The combination of FTIR with CO adsorption, and temperature-programmed desorption of NH3, confirms that the presence of strong but few Bronsted acid sites in the 10-MR channels increases selectivity to MoBUFA. (C) 2015 Elsevier Inc. All rights reserved.