The influence of the free fatty acids, water, temperature, and reaction time on the catalyst-free microwave-assisted transesterification of triglycerides with 1-butanol was studied using the response surface modeling (RSM) approach. Stearic acid and/or water were added to rapeseed oil to construct the model. Afterward, the general applicability of the model was verified using other triglyceride sources (refined oils as well as waste fats and oils), other free fatty acids, and other alcohols. In most cases, the model gives a fair approximation of the actual yield of fatty acid alkyl esters; however, the model tends to underestimate the yield when waste fats and oils are used. Next to that, a significant increase in prediction error was observed when using branched alcohols. For reactions of 2 h at 234-264 degrees C, yields over 90% could be reached when using waste fat streams.