Free fatty acids (FFA) in low grade oil could be reduced by using an esterification reaction with glycerol, which itself is a waste product of biodiesel synthesis to give mono- and di-glycerides. This study investigated, for the first time, the reaction between oleic acid as a FFA with glycerol, using a phosphonium-based deep eutectic solvent (DES) as a catalyst. The effects of temperature (120 degrees C, 150 degrees C and 180 degrees C) and the DES catalyst concentration (1, 3 and 5 wt%) on the esterification efficiency of fatty acid conversion were observed. The glycerol to fatty acid molar ratio (6:1) and agitation speed (600 rpm) were kept constant. The results revealed that the lowest activation energy of 54.64 kJ/mot was attained using 5 wt% of DES catalyst. Accordingly, optimum reaction conditions were found at a temperature of 150 degrees C with a 5 wt% DES catalyst, which produced 95% FFA conversion after 30 min of reaction time. In addition, 85% of combined mono- and di-glycerides was observed at the optimum condition, which will be easier to transesterify using alcohol. The process was repeated without adding any fresh catalyst and results showed that catalyst activity sustained very well, suggesting that it can be reused a number of times. (C) 2017 Elsevier Ltd. All rights reserved.