The transesterification of two vegetable oils containing different quantities of free fatty acid have been compared over a series of BaO/Al2O3 catalysts with a range of baria loadings/dispersions. Dispersion of baria on the alumina was determined by pulse chemisorption of carbon dioxide. Limited agreement was found between the numbers of exposed sites for CO2 adsorption and the reaction rate and the rates measured were different for the two oils. The latter was unexpected as the rate determining step appears to involve only the activated adsorption of methanol, consistent with the change in rate measured when methanol was replaced by ethanol. Differences between the behaviour of the two oils and the lack of correlation between rates and available basic sites can both be accounted for by the strong dissociative adsorption of the free fatty acid which results in a less active catalyst for the transesterification of the triglyceride. Higher dispersed samples show less sensitivity to free fatty acid and give the highest rate per exposed surface site.