The catalyst-free microwave-assisted conversion of free fatty acids (FFA) to alkyl esters in triglyceride feedstocks with a high acid content was studied. To this end, esterification of FFA with higher alcohols was investigated using the response surface modelling approach (RSM). A model was constructed using stearic acid and 1-butanol. The influence of reaction temperature and the FFA content in the triglyceride feedstock were studied by varying both parameters on 5 levels between 175 degrees C and 275 degrees C, and 30 mass% and 100 mass% FFA respectively. Afterwards, the general applicability of the model was verified using other FFA, fatty acid distillates (FAD) from industry and other alcohols. When using other FFA, FAD or primary alcohols other than 1-butanol, the model is able to accurately predict the actual conversion with a typical deviation between actual and predicted data of 2%. After 1 h of reaction at temperatures above 250 degrees C, conversions over 90% were reached. (C) 2014 Elsevier Ltd. All rights reserved.