The possible applications of the native lipase of Nigella sativa seed in oleochemical reactions were investigated in relation to the process parameters. The investigated enzyme-catalyzed reactions were the hydrolysis of used frying oil and esterification of oleic acid with glycerol and methanol. All reactions were carried out under solvent-free conditions and catalyzed by pressed seed as the lipase source. The hydrolysis reactions catalyzed by pressed seed were carried out in the presence of two nonionic surfactants which showed similar effects on the reaction. Esterification of oleic acid with methanol was carried out without removing the produced water. The highest yield of ester was observed in the esterification reaction conducted at 45 degrees C using a 1:1.5 methanol:oleic acid molar ratio and a pressed seed content of 50% based on total weight. Esterification of glycerol with oleic acid was carried out with and without co-produced water removal. Esterification of glycerol with oleic acid carried out without produced water removal was conducted at 1:4.5, 2:1, 3:1, and 4:1 glycerol:oleic acid molar ratios. The highest conversion of oleic acid was observed in the reaction carried out at a 4:1 glycerol:oleic acid molar ratio at 45 degrees C and a 45% pressed seed content based on oil weight. The reaction product contained 15.7% triolein, 15.8% oleic acid, 37.3% diolein, and 31.2% monoolein. The best triolein synthesis was observed at 55 degrees C and a stoichiometric glycerol/oleic acid molar ratio of 1:3 with 45% pressed seed content based on total weight with produced water removal. A final triolein yield of 81.7% was achieved.