In this research, soybean oil was hydrogenated for a period of 100 min at 220 degreesC and 2.5 atm H-2 pressure in the presence of Ni (0.18%) catalyst in a batch reactor. The changes in iodine value, refractive index, selectivity ratio, fatty acid composition, trans fatty acids and solid fat content (SFC) during the hydrogenation were investigated on the samples taken at 10 min intervals. According to the results, refractive index and iodine number regularly decreased during the reaction; however total trans fatty acid content (TFA) increased with the starting from the 10th min to the end of the reaction. Most of cis 18:3 and cis 18:2 fatty acids saturated and converted to dien, monoen or their isomers. When oleic acid began to convert to stearic acid, hydrogenation process was ended. Therefore, oleic acid content could be protected. At the end of the reaction TFA (total trans fatty acids) and TSFA (total saturated fatty acids) reached to 56.76 and 22.3, respectively. On the other hand, SFC values increased during the hydrogenation process. (C) 2002 Elsevier Science Ltd. All rights reserved.