This work reports the production of fatty acid methyl esters (FAMEs) and fatty acid ethyl esters (FAEEs) by means of waste fried oil (WFO) transesterification using methanol and ethanol at supercritical conditions, in a continuous catalyst-free process. Transesterification experiments were performed from 573 to 623 K, at 10, 15, and 20 MPa, with oil/alcohol molar ratios of 1:20, 1:30, and 1:40 and water addition to alcohol of 0, 5, and 10 wt %. The extent of the reaction was explored using a novel parameter, convertibility, which corresponds to the maximum ester content attainable from the feedstock (92.1%). The highest FAME and FAEE contents achieved were 81.& and 82.2%, respectively. Results show that transesterification of WFO in methanol was more efficient than that in ethanol, the temperature had the strongest influence, and the addition of water considerably improved the ester yield. © 2013 American Chemical Society.