A method of producing fatty acid alkyl ester from an organic oil source containing at least one free fatty acid, wherein the vegetable and/or animal waste oil has an acid number of at least 30 mg KOH/g and wherein the method comprises the steps of a) reacting the oil source with glycerol at a temperature, which is at least 110° C. and does not exceed 180° C. during the reaction, in the presence of a catalyst comprising at least methane sulfonic acid or the homo anhydride thereof; and b) acidic transesterification at a temperature, which is at least 110° C. and does not exceed 160° C. during the reaction of the reaction product from step a) with an alkanol; and c) isolating the fatty acid alkyl ester from the reaction product of step b).