Biodiesel is produced by the catalytic transesterification of renewable sources such as vegetable oils and animal fats. It is an attractive alternative to diesel fuel because of its environmental benefits. In this work, the transesterification of soybean oil with methanol using zinc carboxylic salts as Lewis acid catalysts was studied. The esterification of fatty acids was also researched. In both reactions, the stability of the catalyst in the reaction medium was examined. Zinc carboxylic salts of different chain lengths, Zn(CnH2n+1COO)(2) with n = 1, 11, 15, and 17, and Zn(C17H33COO)(2) (zinc(II) acetate, laurate, palmitate, stearate and oleate, respectively) were prepared and characterized by X-ray diffraction, FTIR spectrometry and thermogravimetric analysis. The Zn salts were tested in the transesterification of soybean oil at 100 degrees C for 2h. They presented oil conversions between 88 and 94% with fatty acid methyl ester (FAME) yields between 71 and 74%, and they were stable in three consecutive tests. Zn acetate was not stable. The salts transformed into Zn glycerolate at 140 degrees C in the reaction medium (zinc laurate, palmitate, and stearate only partially). It was observed that, in the presence of fatty acids, the carboxylates of the salts were gradually exchanged for the acid to be esterified. With oleic acid, this process was completed at 60 degrees C for all the Zn(CnH2n+1COO)(2) salts. Zinc laurate, palmitate and stearate are crystalline solids, but soluble in the reaction medium at 100 degrees C, and they recrystallize rapidly at room temperature, presenting certain advantages as regards decreasing mass transfer resistance during reaction, and easy separation and recovery from reaction medium. (C) 2012 Elsevier B.V. All rights reserved.