A real sample of waste cooking oil having an acidity of 8 mgKOH/goil and a water content of 700 ppm was efficiently converted into biodiesel. The process consisted in a two-step reaction: a preliminary treatment with AlCl3·6H2O to convert free fatty acids into the respective methyl esters, followed by alkaline transesterification using KOH as a catalyst. In detail, pretreatment with AlCl3·6H2O (345 K, 4 h, weight ratio MeOH:oil:catalyst of 1:3:0.01) allowed to obtain an efficient conversion of starting free fatty acid into the relevant fatty acid methyl esters (yield > 95%). After the reaction a convenient separation of phases was obtained. Two different phases were isolated: an oily phase with a methanol content of 7 % wt, in which methyl esters were dissolved in, with most of glycerides and with a very limited content of water to one side, and a lighter phase, in which most of unreacted methanol and catalyst together with water were solubilized. Such a separation made possible the direct use of the pretreated oily phase with AlCl3·6H2O in the transesterification without any further treatments, unless to adequate the methanol content. KOH was used to complete the conversion in the second step. Reaction occurred completely in 4 hours and the final biodiesel was conform to EN14214.