Homogeneous aluminium species, obtained by dissolving AlCl3 center dot 6H(2)O into methanol, were characterised and tested as catalysts into the direct esterification of free fatty acids with methanol. The nature and the role of this catalyst was further investigated through ESI-MS and FTIR spectroscopy, by revealing an immediate exchange reaction between methanol and the water molecules originally bounded to the aluminium, producing a final mixed methanol-aquo-complex whose reactivity was found to be comparable to that of a methanolic solution of hydrogen chloride. Reaction conditions were optimised using the desirability function applied on the response surface methodology analysis of a Box Behnken factorial design of experiments. By carrying out the reaction at 72 degrees C for 120 min and using a catalyst amount of 1.5% (mol of Al respect to fatty acids), almost 94% of the starting acids were converted. At the end of the reaction, a biphasic system was obtained in which the upper methanolic phase, which contained most of the starting catalyst, was separated from the heaviest phase, mainly composed of fatty acid methyl esters. Such a distribution not only allowed the biodiesel to be easily separated, but also the catalysts were efficiently recovered and reused for at least four times, determining a total TON greater than 200, without revealing any loss of its activity. (C) 2015 Elsevier B.V. All rights reserved.