The transesterification of the lipid fraction of grease trap waste in a catalyst-free medium, under supercritical conditions, was studied. The aim was to evaluate the effect of different concentrations of water (in ethanol) on the production of ethyl esters. Reactions were conducted in a continuous reactor applying different temperatures (275 degrees C, 300 degrees C, 325 degrees C) and residence times (10 min, 20 min, 30 min) using ethanol with water concentrations varying from (2.5 to 10) wt%. The addition of up to 2.5 wt% of water favored the formation of esters at low residence times. The ester yield was not affected by longer residence times (30 min) and with a water content of 5 wt% or higher the ester yield was reduced. It was found that a high temperature (325 degrees C) and long residence time favored the thermal degradation, which was more prominent for ethyl linoleate. The highest ester yield (92.6%) and lowest content of unreacted compounds (5.5 wt%) was obtained using ethanol with 2.5 wt% of water.