Abstract The purpose of this paper is to characterize biodiesel produced by two-step alkali transesterification of waste sunflower oils (WFOs) using methanol and KOH as catalyst and at the same time to investigate the influence of different purification methods on the properties and yields of the obtained products. Three different methods have been chosen for the purification of the product mixture after the transesterification reaction: washing the mixture with (a) silica gel, (b) 5% phosphoric acid, and (c) hot distilled water. The physical and chemical properties of the used feedstocks and the obtained biodiesel were determined and compared in order to investigate the effects of WFO properties and purification methods on the product characteristics and yields. In general, it could be said that properties of biodiesel reflected the ones of the feedstocks. The produced biodiesel met the criteria required to be a diesel substitute with respect to the density at 15 °C, kinematic viscosity at 40 °C, acid values (except in the case of biodiesel obtained from WFO with acid value higher that 3 mg KOH/g), iodine values, the content of methyl ester of linolenic acid, and the purity. The biodiesel yields were inversely linked to the acid value and viscosity of the feedstocks. Results showed that silica gel and phosphoric acid treatments gave the highest (∼92%), while the hot water treatment the lowest yields (∼89%), recommending the silica gel and phosphoric acid treatments as more suitable for the purification of crude methyl esters obtained after the conversion of waste frying oils by two-step alkali transesterification reaction.